Very interesting how you managed to develop a platformer with just right and left controls, and still make it work. Congratulations on the idea and the effort!
The red and blue blocks were indeed the most difficult, but I believe it was due to the unpredictability of knowing whether they were solid or not. At several moments, the red block seemed solid, but when advancing towards it, the character would fall. One idea to make this part easier could be to make the blocks appear and become solid abruptly, but disappear gradually, and once they have fully disappeared, keep them solid for a short additional time to give the player extra time to correct small delays.