This was Greydon Clark's first movie and it comes a bit late in the blaxploitation cycle and is largely forgotten nowadays. Clark would go on to direct the awesome 80's videogame movie called Joysticks in 1983.
Black Shampoo is definitely in the "Shaft" and "Slaughter" school of blaxploitation movies but it sort of veers into Jack Hill-esque ridiculousness. Highly recommended. It's available from VCI Video on DVD.