GRS 1915+105 is almost certainly stellar in origin, despite its being the largest stellar black hole in the Milky Way that we know: there is also another star orbiting it, too.
But I have also noticed the screenshot of a Yahoo page where the problematic ad appeared. It is about dark gulping.
See Google News, Yahoo/Space.com, Softpedia, MSNBC,claiming that superlarge black hole holes - with billions of solar masses (such as those found at large galactic centers) were created by a collapse of dark matter.
It seems obvious to me that such things can take place. Gravity only takes about the total mass and momentum, not the "color" of the matter; the only question is how often these collapses occur.
There is more mass stored in dark matter than baryonic matter. On the other hand, dark matter tends to be more diluted. I am not sure which difference wins - and it is better for me at this moment to trust the simulations of others. But at the end, the number of collapsed-dark-matter black holes could be comparable to the black holes of baryonic origin: but they will be substantially larger.
When they're already created, you can't tell them apart. Recall that black holes have no hair.