Hi designer,
Welcome to Affinity Forums :)
Yes, both apps can do this easily (there's also various methods to achieve it). In Affinity Designer you can simply draw a black rectangle over the image (with the same size of it extending just the side you want - enable snapping to help you align the two), then use the Transparency Tool (the glass cup icon) to apply a linear transparency to the rectangle dragging the tool over the rectangle on the canvas. Adjust the midpoint (the small vertical line in the middle of the transparency handle) to control the "feathering transition" from the black rectangle to the image. Let me know if you still have trouble.
