My experience indicates the nonuniformities and grain on stretched canvas noticeable detracts from the viewing resolution. If you use canvas, you need to paint it to remove the non-uniformities, but the painting process tends to be non-uniform. Even said plain cotton duck canvas looks OK for starters.
Blackout material comes in different types and any given fabric store usually has a small selection. You have to decide what makes the most sense. I have seen white vinyl blackout that would look very good for a white matte screen and definitely better than canvas.
I was lucky to find a stretchable raincoat vinyl coated poly that makes a nice screen on the cloth side. The density of the weave is much better than canvas and barely noticeable. I like it the best because it is completely flat, extremely uniform and slightly grey so it has good color saturation.