In a standard deck of 52 cards, there is 4 cards "seven", 2 of them are red and 2 are black.
So, from the 52 cards, only 2 are black seven. Thus, the probability will be the 2 we want to draw over all the possibilities, that is, 52:
[tex]\frac{2}{52}=\frac{1}{26}\approx0.03846=38.46\%[/tex]