You CAN have a black hole made of only photons. Of neutrinos even.
You can indeed (and such black holes are even more cool). However I believe the formation of a black hole from photons requires that the photons aren’t all propogating in the same direction. This is a good thing because if you did have a black hole traveling at the exact speed of light then, well, reality is all broken down. Black holes have rest mass; speed of light travel is off limits. In general it doesn’t really matter (so to speak) whether the mass-energy that they are created from is photons or a mix of paperclips and left over Babyeater offspring.
I tentatively suggest that there may be an error in a detail here, albeit not the one shminux suggests.
You can indeed (and such black holes are even more cool). However I believe the formation of a black hole from photons requires that the photons aren’t all propogating in the same direction. This is a good thing because if you did have a black hole traveling at the exact speed of light then, well, reality is all broken down. Black holes have rest mass; speed of light travel is off limits. In general it doesn’t really matter (so to speak) whether the mass-energy that they are created from is photons or a mix of paperclips and left over Babyeater offspring.