The Illustrator blob brush tool can be found beneath the paintbrush. Or access using shift+B as a shortcut.
Go to the paintbrush and select the second entry 'blob brush' and set the color of the fill. The color can be a solid color. The color can be a gradient. Double click the icon to display the options or press return. Apply stroke to the art-board. Instead of stroke, it applies as a vector fill. Select the vector design, the design is made up of multiple points. The result can be manipulated using the other features of the application, such as changing the opacity, changing the blending mode, adding additional points, removing points from the path etc
Access the settings by double-clicking the icon. You can also press return. The panel offers 'Keep selected', 'Merge Only', fidelity, size, angle, and roundness, along with randomness and pressure settings. It is similar to the eraser. It is also similar to the basic Photoshop stroke settings.
Two check-boxes are available. To keep the path selected, check the Keep Selected button. If not, you can always select the path manually. If merge only is turned off, any new strokes are added as new paths. If merge only is on, all strokes are combined. You can always use the pathfinder panel to unite the vector paths. Fidelity allows for more accuracy but adds more points. Smoothness is less accurate but fewer points. Set the size, it can be a thin line or a big bold stroke. Set the roundness to less than 100% to create an elliptical stroke, more akin to a calligraphic stroke. With roundness less than 100%, you can also angle the stroke.
There are also a few additional options such as having a fixed size or introduce randomization and variation for the size (as well as angle and roundness) as well as settings for an art pad and pen combination. Click OK. It should be noted that the randomization is per stroke and not sadly during the actual application of each stroke. The randomization is limited to the size so if you set it to 20pt then the randomization will be limited to the 20pt as well (which I must admit I find quite frustrating). Same for angle and roundness. It would be great if there was a randomization of color available as well but there is not though you always just change the color via the fill color as you add the artwork. You can also randomize the result by using the color guide.
The generated path can be filled with solid colors such as yellow and red. Use a gradient for a special touch. Or a pattern. Color effects can also be added. The color can be changed at any time. Additional fills can be added via the Appearance panel
Hold down the shift key to apply it in a straight line. You can create multiple straight lines. Zigzags also, angle your strokes at forty-five degrees. Squares and rectangles, angle your strokes at ninety degrees. Many combinations of shapes such as lattices can also be created.
As you add the artwork, you will only see the fill being added but you can always add a stroke on completion as well as other effects such as live effects / blurs / additional fills etc via the appearance panel. You can also then add width profiles to the path. You can also add effects such as as offsets You can also add loads of new fills and add transformation effects to move the fill to a different position or simply use blending modes to blend the colors as well as the strokes etc. Apply it and select the created path and set a stroke color such as black as well as a stroke width / weight and then go to the width profiles and add one or use the width feature to manipulate the stroke.
You can also merge it with existing paths. Double click the icon and set the merge to ON and keep selected ON and then go select a path such as circle or square and then apply it to that and instead of create a new path, it will be added to the existing path. If you select multiple paths and they are different colors then this will not work. It will only work if it a single color path. If you have 10 paths and they are all separate and they are all orange and then you apply it, all will be merged into a single path with the path connecting between all the shapes. If one of the paths is blue and not orange, it will be ignored.
A major issue with this is that it does have tendency to create a lot of extra points especially around slight creases in the stroke, so if you create a perfectly straight line with it then you won't get too many points but the slightest move away from that at the corners of the stroke and you will see a bunch of lots of additional points. You can use the applications' simplify feature but often that seems to end up making it worse. My favourite is the smart removal from Astute Graphics and that will get rid of a lot of the excess points and also keep the design looking much the same as before.
You can use the strokes to create other presets such as symbols as well as swatches Simply add a path and drag to the symbol panel and select item in the symbol panel and use with the sprayer. You can also do the same with the swatches if you wish to create a tile and you can use it in the pattern editor.
The result can still be added to using the same features of the tool to merge the fill. With the latest release, if you add the artwork as a symbol preset then you can also now add it as a dynamic or static item and if added as a dynamic then you can use the artwork as a master and add color and strokes etc to the instances added to the art board.
You can also use it with the repeat feature that has been added to the recent release of Adobe Illustrator. You can use it with the radial option as well as grid and also mirror to create all kinds of amazing designs. You can increase the number of instances, you can change the distance of the strokes from the center, and much more. The spots are still live. You can edit them by double-clicking the repeat object. Change the size, angle, color, etc of the design. Select 'Exit Selection Mode' to leave.
Personally, not sure why it is not on the same grouping as the eraser as it is basically the same just inverted. The eraser can be added across the applied brush stroke and break the art into new individual paths which can then be added to by additional strokes.
Once you have added the artwork (and you can add them as actual spots or in one continuous stroke) you can add effects such as tweak or roughen or offset or glow etc and you can then expand the appearance via the object menu. Or add graphic styles
The paths can be filled with nil, the stroke set to black and a quick transform applied to create a lovely zoom transform effect. Sadly, you can't add to the artwork once you have added the appearance. Create the design and then apply the effect. Use the shift key to control / constrain the angle to create zigzags etc. Effects can be added such as offsets to create all kinds of unique designs from a single stroke