Affinity Photo's Maximum Blur Filter can be found in a sub menu of the blur category (found in the filter menu). It is a destructive effect that can be used with images as well as resources such as fonts and brushes etc. It blurs and intensifies the white / lightness of an image resulting a smudge or sparkle to any image.
It comes with only two settings radius and circular. If you select circular, the effect is circular (dots) or the effect is square by default. The radius can be set to 0 (no effect) to 100 (total white out though this does depend on the size of the image). Any white in the image will be blended into the darker parts of the image. If you use a low setting of radius then the result will generally be a sparkling effect. If you use a very high setting then the result will be a far more white smudged image.
The tool is easiest to use by duplicating the original image and then using the effect on the new layer. If you use the effect on a layer, you can delete the layer but you can also combine the layer and effect with blending modes and opacity. There is no live layer, so the layer approach is closest to the live approach. You can also create interesting effects by duplicating the layer and applying the effect and re-sizing the layer (as well as rotating).
It can be used on its own. It can also be used with others such as Gaussian. You can use the effects before or after the maximum blur to create all kinds of unique images. It also works amazing well with live adjustment layers so you can add channel mixer adjustment layer to modify the color.
You can use the channels panel with the effect. You can add different effects and settings to each channel. Channels panel. Select red channel. Apply to the red. Select the green channel and apply with different settings. Select the blue channel. Apply with different settings. You can then merge them all back to a single RGB. You can also use the same effect with adjustment layers such as black and white adjustment layer. The channels option does allow for the interesting creation of edges in colors (if using different radius settings)
Create some text (in black) and then merge down. Use the effect on a white background and the text will thin as you increase the radius setting. If you use the same text but as a layer with the effect, you will see black text start to thicken as you increase the radius (using either square or circular). Create some text using the artistic text tool and set to black. Apply with a low setting. The text becomes much bolder and nearly unreadable. Create some text using the artistic tool and set to black and Merge down via the layer menu with white background. Apply with a low setting (if set too high the text will just vanish). The text is a lot thinner. Of course, if you are using a black on a blue background the result is totally different. You can also create combinations with effects such as Gaussian and the result is very abstract and alien like.
You can use the rectangular marquee tool or elliptical etc as well as any of the other selection features of the application to apply the effect to only that region (or the inverse of). You can also use the select menu and create a tonal (mid tone) etc selection and then use a feather with that such as 100px etc and that creates a surreal painted like effect with images instead of a full on blast of white.
You can also use it with a layer or image and then go to the edit menu and use the fade command. This allows for the processed image to be modified by a blending mode and / or opacity.