Thanks for your answers so far, I'm sorry I'm still being dim - the 'mass gaps' per the Wikipedia article you linked are thought to be in the region of "2 to 5

Thanks for your answers so far, I'm sorry I'm still being dim - the 'mass gaps' per the Wikipedia article you linked are thought to be in the region of "2 to 5 and 50 to 150 solar masses", and therefore stellar black holes are expected to be around 5-50 solar masses. The NYT article puts Cygnus X1 at 21 solar masses and therefore safely in the realm of possibility, but apparently now big enough to cause a problem, being bigger than 20 solar masses which was some other previous limit (the limit for a binary system?). I don't understand why it couldn't have been <= 20Msol at formation, and then acquired the rest parasitically from it's companion.