Online blackjack is one of those games where it is very hard to be able to get a feel for the game and have an idea of what your opponent may be hiding from you. Since you cannot see what they are doing or how they are going to play, you cannot make decisions on how to play your hand. The game of poker is quite similar to blackjack in this way. You can often get an idea of what the other player is going to do and then make a decision about your hand. When playing online, though, you do not have this advantage.
Keeping track of the cards on the table as well as watching how others are betting will give you a leg up on the competition and provide you with a way to get an idea of what they are likely to do. Still, it will be in your own skills that you either win or lose with online blackjack.