Blob brush tool. The blob brush in Illustrator CC CS6 etc can be found beneath the paintbrush tool or use shift+B to select the tool. The tool uses fills and not strokes. Go to the paintbrush tool and select the second tool 'blob brush'. Set the color of the fill for the blob brush. Double click the icon of the tool to change size settings etc. Apply tool to artboard. The blob brush is a wonderful tool for some really unusual art in Illustrator.
You can use the Illustrator blob brush to create tangled artworks as well as flowing lines as well as hollow line designs (just set the fill to none and set the stroke to black and 10pt etc. You can use it to create quick zigzags. You can use it to create quick grids. You can use it to create basic lines. You can use it to create lovely flowing strokes and drawings. You can use to create blots and dots and splatters (the size of the blots can be made to vary via the randomization of the size of the applied stroke) and much more to the artboard. If you want to use the tool to create a quick grid then use the shift key to constrain the drawn artwork to a straight line. The easiest way to create the grid with the tool is to use the rulers via the view menu and use the rulers to create a line from top to bottom and then duplicate that line by holding the alt / option key down and create multiple duplicates of the line all shifted a certain distance from the original line. Repeat the same from left to right and duplicate that line created by the tool. You can then select all the generated artwork and using the pathfinder tool, combine all the paths into a single path via the unite button.
Set the randomization by double clicking the tool icon and then going to the size field and instead of fixed. You can set the settings from fixed to random and that gives you the option to set a variation in size. If you choose a large blob size then you can also set a corresponding large variation (I must admit, it does slightly bug me that you can't set a low size and a large variation as you may like a default 5pt but for that to occasionally jump to 60pt - well you can't).
You can see the settings for the Illustrator blob tool by double clicking the tool icon or pressing return while you have the tool selected and there are a number of settings to be altered and many different results can be achieved by subtle changes to the settings : Keep selected + Merge only + Fidelity + Size + Angle + Roundness. Set the merge only with selection to OFF via the tool's options. You can then apply the stroke to the artwork and pass through objects on the artboard and they will then be merged with the current stroke.
The key thing to join paths is that the paths have to be the same color. If one of the paths is red and one is black then the blob brush will ignore the Illustrator paths. The size and angle of the blob brush etc are great for creating calligraphic brush strokes of varying dimensions. 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 artpad / pen. The tool's result can be filled with gradients as well as solid color and also patterns.
You can use the appearance panel to add fills and strokes as well as effects such as blurs and more to the blob brush artwork. 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 fill the artwork with styles via the styles panel.
You can also then add width profiles to the Illustrator blob brush. You can add dash styles and much more. You can add the vector path generated by the tool in a straight line by holding down the shift key as you use the tool. You can constrain the generated line to 45 degrees and 90 degrees. You can repeat apply the lines and generate a series of parallel lines. Depending on the selection set for the tool, you can merge with other paths already created (circles etc). Select the tool and them double click the tool icon and make sure that the merge and selected are set to ON. You can then quickly apply the strokes to paths such as circles or rectangles or any other paths to create interesting variants of an existing design such as creating distressed works or frames and more.
A quick and easy way to create a frame design is to create a quick rectangle or circle and then use the blob tool with that path. Turn the merge with selection to OFF. Apply strokes out from the circle or rectangle. Set the size to 50pt and set the variability to random so the lines coming from the path will be varied in size (unless you want them all to be the same size).
The tool is also useful for creating variable sized spots across the artboard by using the tool with a size of 50pt and variability set to 12px and click in different locations throughout the artboard. Don't move the stroke, just click to create a spot. There is no color variability so if you wish to vary the color then change the color all the time or do this after you have added all the dots or use the live paint bucket to fill them all with different colors (select all the paths and click them and change the color via the fill setting).
How to create new brush presets using the blob ? Select the blob tool and apply the strokes. Once you have created your spots and dots with the tool, drag the end result to the preset panel and save as art or scatter. The blob brush can then be used to create millions of unique designs for use with standard stroke tools.
See video tutorial on subject