I wrote a simple upscaler that replaces each 3 x 3 block in an image with the best matching nearby 6 x 6 block. No color shifts or thresholding is performed, and block transforms could probably be skipped too. Searching is limited to the 121 blocks around the block to be upscaled.
Blockiness aside, it definitely sharpens high-contrast edges. As with Perfect Resize, one can see the “oil painting” effect start to come in. The image below was upscaled from 510 x 510 pixels to 1020 x 1020 and then cropped slightly.