One of the most likely candidates as the early roots of blackjack is the 17th century French game Vingt-et-Un, or "21". The rules were a bit different from modern blackjack; only the dealer was allowed to double, and there was a betting round between each of the cards dealt.
Another similar game is the Spanish "One and Thirty," with the goal of reaching 31 with at least three cards.
Blackjack in North America
The game followed European immigrants to North America in the late 19th century, and made its way to Las Vegas, Nevada when gambling became legal in the state.
At first, the game wasn't very popular, as most Americans preferred craps and roulette. To attract players to the game, the casinos tried different bonus schemes to make it more attractive.
One such bonus paid 10 to 1 and was paid out when the player made a 21 with the ace of spades and a black jack (the jack of clubs or the jack of spades,) and the name "Black Jack" was invented.
Blackjack started to grow in popularity, and by the 1930's the game was the third most popular game in Las Vegas. In the 1950's it was second only to craps.
As blackjack became increasingly popular, mathematically proven strategies for the game started to appear. The first notable book on blackjack strategy was Baldwin's "The Optimum Strategy in Blackjack," which was followed by several others.
Soon, mathematicians discovered the possibility of improving their expected value by counting cards in blackjack and even "beat the dealer," (also the title of a blackjack book by Professor Edward O. Thorpe).
Thorpe was the real pioneer of blackjack strategy, and paved the way for many card-counting blackjack players during the second half of the 20th century. As even more complex strategies were developed with the help of computer simulations, skilled players - or even teams of players - could take advantage of the statistically proven theories and make lots of money before they were banned from the casinos.
Today, with a multitude of online blackjack sites to chose from, blackjack is not only one of the most popular casino card games of all time, it's also played by millions of players online on a regular basis.