The custom blur filter in Affinity Photo can be found in the blur category of filter menu, it is a powerful destructive effect and can be used with images as well as combined with other resources
It is not particularly the most user friendly filters as all you can do is enter numbers and there is no presets to store the settings any way which is slightly annoying when you have entered 10 or 12 numbers into a panel but at least it does remember the settings from panel session to session ! You can use it to blur (though to be fair, it is quite tricky to achieve that sort of result) as well as fade images and much more but it is one of those features that does require some patience. You can control it by entering 0 1 2 3 etc as well as -1 -2 etc but it does not make for any easy way of working, though only apps that have this feature work in the same way. You have the option to add in a divisor and offset as well as normalize or not (if turned off, you can modify the divisor).
The offset field works in an unusual way, in some images you can enter 1 and the image vanishes, in others you can enter 100 and it may just lighten the image, the result depends on the entries in the grid, increasing the setting generally results in more brightness. You can enter decimals into the offset such as 0.0001 etc. 100 seems to be the limit for the grid as well as the setting fields, -100 is the minimum though you can enter 0 etc as well, don't assume that 100 or -100 will result in anything different in some cases. You can also create all kinds of transparency in images by using combinations of settings. Unfortunately, there appears to be no reset and cancel still leaves the settings from the previous attempt (which is odd as many other effects always reset each and every time). Trial and error with this does require some patience.
Even though the tool is called one thing, it can also do a little bit of unusual sharpening depending of the settings as well as some pretty unusual embossing by entering large values in the second row.
offset 0.9, divisor -8, 10, 0, 0, 0, 10, 0, 0, 0, 0, 0, -40, 0, 2, 0, 0, 0, 0, 0, 0, 0, 10, 0, 0, 0, 0, 10
offset 0.9, divisor -8, 10, 0, -70, 0, 10, 0, 0, 0, 0, 0, -80, 0, 2, 0, 80, -50, 0, 0, 0, 0, 10, 0, 70, 0, 0, 10
offset 1, divisor 3, 0, 0, 0, 0, 0, 0,0,0,0,0,0,0,7,0,0,0,0,0,0,0,0,0,0,0,0
offset 1, divisor 3, 0, 0, 0, 0, 0, 0,0,0,0,0,-10,7,7,7,-10,0,0,0,0,0,0,0,0,0,0
offset 0, divisor 59, -50, 0, 50, 0, -50, 0, 40, 100, 0 , 0, -50, 0, 1, 0, -50, 0, 0, 0 , 0,0, -50, 0, 50, 0, -50 (sharpening)
It can be used more than once as with most of the tools. You can apply it and then apply it again using the repeat command. If you apply a fading effect to the image or text then the result of a repeat will be even more fade to the point where you can barely see it.
It can also be faded via the layer menu and you can then fade it to 50% or perhaps change the blending mode for the effect to perhaps use darken or difference or a lighter color and these settings can be super useful.
It can also be used with multiple layers and different settings for each layer and also combine those layer with different blending modes. You can use layers to create all kinds of frame designs as well as collages etc with this effect
Selections can also be used to limit the range of the custom blur effect. You can create all kinds of selections using the marquee tools as well as the select menu feathering, grow etc and then apply this to those or the inverted
You can also use shapes as well as type with the effect. You may find it works best with text merged into the document via the merge selected or merge visible.
As well as combine it with pattern layers, though it would be a good idea to set the mirror on via the move tool and then use the effect with the patterns otherwise you will end up with sharp seams appearing in the design
Can also be used with the channels panel and applied to the red only etc
It has to be the most frustrating tool as you cannot even reset all the 16 entries (as well as the normalize) to the defaults so if you have -40, -3, 20, 23, 1.23 etc in the fields you just have to manually enter the values of 0, 0 etc. It is a pity that there is no preset feature with this as you have to recreate it over and over, though a quick way to save it as a macro and restore the settings that way but it is not ideal, you still don't have access the actual settings so you can modify them though