As far as I can tell there is no way to equip the black blade when making a bladebound magus.
Blackblade is a companion you can make. You choose the weapon to be the black blade, and then equip any normal weapon of that type. The benefits for the weapon will be applied to the weapon. The companion is treated as any other companion for referencing. I did not code up black blade weapons, since that would mean making a copy of every single weapon. If you have any issues with the features not working as I described, let me know. Please cancel this issue if that makes sense and works as expected with that information.
Closing this out in one week if I don't hear back from you.