Why is my dentist ignoring black marks on bottom 2 teeth?!

The tarter build up will stain over time. If it's on the chewing surfaces of your teeth it is hard to get the staining out. The reason it's there is you let the light plaque stay on your teeth over a period of time causing it to harden. Make sure you're really brushing your teeth well. Have your hygienist try and use an ultra sonice to get it out but it may not come out all the way. Also show the hygenist how you are brushing to make sure you're doing it effectively.

I'm a dental hygienist

Go to a hygienist, there is one in mostly every dentist and they will scale it off, don't leave it anylonger have it removed ! ;)

Calculus turns black after its been a long time and has undergone long term calcification.
Go to another dentist and get it scaled off.