It means he showed a hostile, angry, or irritated expression on his face. It's called a "look" because it involves wrinkling up the muscles around the eye. "Black" doesn't mean a color, it means a dark emotion: anger, or at least annoyance and irritation. The salesperson hoped to sell the speaker an expensive new car. The speaker said "I'm really looking for a six-year-old used car" or something like that. The salesperson thought to himself "I'm not going to make much money on this. I'm angry at this customer. I wish this customer would go away and not waste my time. I'd like to get rid of this customer and try to find a better sales prospect." The salesperson didn't say it, he has to be nice to the customer no matter what he feels, but he let it show on his face. He gave the customer a "black look."

It means dark looks, or unhappy looks. The person giving black looks is displeased by the person receiving them. Hope this helps!

I'm 46 years old and have never heard that expression in the US.