It's a bit of a shady suspect I think.
Asfar I recall, BLM's draw certain powers from the Void/ World of Darkness equivalent in FFXIV. Nald'thal 'is' the guardian deity of the Underworld, therefore, it 'could' be said that all BLM's must, in some way (whether knowingly or not), be connected to that.
Therefore, you may not need the connection to Ul'dah, but you certainly need the Nald'thal bits in some regard.
Asfar I recall, BLM's draw certain powers from the Void/ World of Darkness equivalent in FFXIV. Nald'thal 'is' the guardian deity of the Underworld, therefore, it 'could' be said that all BLM's must, in some way (whether knowingly or not), be connected to that.
Therefore, you may not need the connection to Ul'dah, but you certainly need the Nald'thal bits in some regard.