how did general pershing get his nickname -blackjack?

Question

he was general of the armies for the usa during world war 1.....

Answer 1

There are actually several stories as to how General Pershing got the nickname "Black Jack"; none of which has been positively verified (see first link verifying historical confusion). Here is the run downs of the more popular ones:

1 - Nicknamed for his mean spirit while at West Point (this is the oldest explanation - see 2nd link below)
2 - Received for his leading of the famed 10th Cavalry Buffalo Soldiers (3rd link - this is the most quoted)
3 - Nicknamed when he began teaching African-American students after his graduation from secondary school in 1878 (4th - the rarest explanation)

Answer 2

He commanded a troop of what was then referred to as "colored" soldiers during the Spanish-American War. The nickname was intended to be pejorative.

At that time, black units had white officers and it was an army (and society) prejudice that the black units were given substandard officers; if a man was any good, he'd never get sent to a black outfit.

Other, ruder nicknames were applied to these officers, as well.