Black tomatoes won't turn red, instead they go from green to black.  But the fruit of black tomato plants generally doesn't get completely black when ripe; it's usually more of a purple or maroon color. 

There are other ways besides color to tell if a tomato is ripe--first, when you hold the tomato and squeeze it gently it will yield somewhat when ripe, and be firmer when unripe.  You also might give it a sniff, since ripe tomatoes are more fragrant than green ones.  Ripe tomatoes also feel rather heavy for their size.  If still in doubt, just cut open and taste the most likely looking tomato, to see if it's ripe or still needs some more time.

Happy eating!

Boulder County Master Gardener Desk