Filters menu in Affinity Photo and the Blur category as well as via the new live filter layer menu in layers.
Set radius and click the apply button. You can also change the effect's intensity by interactively dragging across the document. You can set it to a low value or a high value and anything in between to create subtle image effects as well as totally wiping out the image. If you go to the live filter layer, you also have the option of blending mode and opacity to enable additional control as well as a non destructive effect with any image.
It can be used in all the color channels or a single one so you can modify the effect for the red to the green etc. you can apply it once or multiple times to each channel or totally ignore it for another channel. You can apply different radius settings for each to create all kinds of different colorful artwork. You can also combine it with other filters such as the deform filter. You can find the channels panel in the view menu and studio.
You can also use it with a selection, you can apply the effect multiple times inside that selection. It also works amazingly well if you apply a selection feather and then apply the effect to that. Selections can be any shape, they do not have to be rectangles and circles etc
You can fade the filter via the layer menu and fade command. You can use this to change the blending mode for the effect and changing it to difference or darken or lighten can greatly enhance the effect or create a subtle change. Just move the slider to the right or left and experiment.
You can apply the effect to shapes as well as layers as well as text to create all kinds of amazing designs. If you apply the effect via the filters menu then the result for a shape will be a conversion of the design into a pixel layer but it will remove the sharpness of the shape. The same for type. If you use it with standard layers or layers with fx effects then the result will still be a pixel layer.
You can use it with masks. Select some text and then goto the layer menu and rasterize to mask and then apply a radius of 10px etc on it to create sort of ghostly text mask. You can then drag that mask into an existing pixel layer, perhaps invert the mask as well and create all kinds of awesome designs with the original image / mask combination. Perhaps create multiple mask layers and they can have all different settings applied and they can then be blended and modified by distortions etc as well. You can use that effect with all kinds of pixel layers such as images or gradients etc
Select an image or layer or create a selection from it and then goto the layer menu and create new pattern layer from that selection and then re-size it using the move tool. You can also set the mirror on in the top left and that probably works best if you use this filter. You can, of course, combine it with many other filters such as median, bilateral etc
The effect can be recorded. You can apply it once or multiple times or combine with other effects such as distortions as well as use it with channels etc and all those steps can be recorded via the macro panel. Click the record button and then apply and set a value etc. You can then add others. Once you have finished, stop recording and then click play to play it back and apply the effect. You can also click the little add to library to save it for future use to the library. You can also click the little gear and save the radius parameter so each and every time you run it, the value will pop up and you can change the setting. As you can create multiple effects and all could have similar names, you can give each of the parameters a name and they will appear when the panel appears.