I keep stocks of both black & white sesame seeds in my pantry & use both frequently. As far as I'm concerned, it's just an aesthetic thing. I've tried both totally plain & don't taste any difference in flavor. But the two of them do look very attractive together in certain recipes where appearance counts.
I don't know the answer for certain, but my belief is that it is only aesthetic. Black sesames are also more expensive and less available.
When I have a recipe that calls for black sesame seeds I always use white.
I'll be interested to hear if there is any other opinions about this out there.