The Bishop is not currently attacking the Black king as he is not in the way of the Bishop's path. On the other hand, the bishop cannot travel to the square a1 as the white king is already on that square.
The Bishop:
Bishops move in diagonal lines (sideways) As long as there are no pieces blocking the way, a Bishop may travel as far as it wishes to. A Bishop in value is equal to 3 points - making it the equivalent to the power of a knight. Bishops capture the same way they travel. Refer to the diagram on the left to better understand about the movements of the Bishop.