How to create PNG files optimized for being compression-friendly? I.e. would it help if image is created (mostly) from 8x8 unicolor blocks?
PNG format is lossless format where for compression the image is first "filtered" and this filtered image is then passed to DEFLATE lossless compression algorithm. The purpose of filtering stage is to make the image more compressible by DEFLATE and current method uses delta-compression from previously decoded pixels.
So if your plan is to pre-process the image in some lossy way, then if you can reduce the deltas so that the following entropy encoding can benefit from it, then you get better compression ratio. For example "blockifying" the image as you suggested would only require more storage for the top-left pixel in the block while the deltas for the remaining pixels would be zero, which DEFLATE can compress very efficiently. This same would apply to any continuous constant color regions.