There are 52 total cards in a deck.
26 of them are black.
Probability of 1st card being black: 26/52 reduces to 1/2
Probability of 2nd card being black: 25/51
Probability of 3rd card being black: 24/50 reduces to 12/25
Probability of 4th card being black: 23/49
Probability of 5th card being black: 22/48 reduces to 11/24
Probability of getting all 5 black cards:
1/2 x 25/51 x 12/25 x 23/49 x 11/24 = 253 / 9996 ( about 2.5%)