Cotton WILL wrinkle. Poly blends are designed not to wrinkle so much.
The best black fabric to hold it's color is Kona Cotton Black, in my opinion. It is thread dyed rather than over-dyed after weaving. But, that is no guarantee of keeping perfect color in constant washing conditions. The water you use, the detergent, the method of drying, how often and how you treat for stains, will all affect the color and condition of the fabric.
Jan in VA