Counting cards in Blackjack is legal or illlegal?

It is legal. You are not going to have to worry about any casino throwing you out, either. Not for a long time. That's because if you try it, you almost certainly won't do it right at first. If you ever get good enough at it where you become a threat, then you'll have to worry about how to not get barred or limited in some other way by a casino.