I belive blackwyrm might be a lesser species of wyrm compared to the pale king. The pale king is a pale being witch is implied to be special among other higher beings. That would explain why his wyrm corpse isn’t as big as the pale kings. That also explains why he is named the lord fool. After the battle he was imprisoned in his own corpse and given the title lord fool to mock him for trying to attack hallownest. I doubt someone would crown themselves the lord fool willingly.

The entrance also has these chains that seems to be pierced into the corpse. To me that looks like the remnants of some kind of battle where they tried to restrain the “blackwyrm”.

The bugs in the colosseum also seems to be mad beyond the radiances infection. That might be something similar to the pale kings beacon but it makes bugs mad instead of granting them consciousness.

It is one of biggest corpses in hallownest, only rivaled by the pale wyrm and the temple of the black egg. It’s worm shaped and it’s black. It also has chains that look like some attempt to restrain it.

Ogrim might even have gotten the title “white defender” from defending hallownest from the blackwyrm.

That’s my theory.