Now that I've played with blades for virtually the last decade, I believe that they are the best type of irons to improve your golf swing. I recently came across this promo video for the new Mizuno MP-4 forged muscle back irons where PGA Tour Pro Charles Howell III expresses a similar sentiment at the 4:47 mark:
If my son decides he wants to play golf...I would start him off in a set like this [muscle back blade irons]. You can tell right away when you hit is solid. You can tell right away how to draw the ball [and] fade the ball. This iron right here, if you play golf with this every day, you'll actually get better. It will make you a better player.So, if cavity back irons are the "game-improvement" irons, then muscle back blades are the "swing-improvement" irons!