This is an interesting conundrum. We've got a few options listed here:
--10 upgrades: +5, 10MP buff that makes it deal +7-level damage to enemies it can drain
--4 upgrades: +5, 10? MP buff that makes it deal +6-level damage to enemies it can drain
--12 upgrades: +6, 10? MP buff that makes it deal +7-level damage to enemies it can drain
And we can go presumably go cheaper if it's an anti-human weapon. If we go this route, the 10 upgrade one seems best, making it human-only so it's a tool for nuking Spar and Dorren. This is a spear mostly for Trence, and one which won't affect our general adventuring much at all. In most cases it'll be a regular old +5 spear, which happens to REALLY devastate any human mages we encounter.
The other option here is mana restore. If the spear restores 3 mana to Trence per hit he lands, that's some serious mana regen potential. It's only four hits to largely pay for itself, which hasted Trence can do in two rounds, including the buff action; in certain cases, it might be able to significantly restore his mana. Wave slashing a group of four twice would be expected to restore 24 (!) mana in one round--numbers high enough to make regular dragon usage entirely reasonable. But... it relies on enemies having mana, and stops charging Trence once they run out. If the average fight only has 10-15 mana available for Trence to drain, all it's really doing is depleting enemy mana without a cost. When we face the Concordat, this would likely be a weapon for Alice, since she gets a lot of value from harrying Dorren or Arngrim, and small mana restores won't be something we care about.
On the upside, it's definitely useful for one or the other; if the mana drain is low, Trence can get big mana restores out of it. If the mana drain is high, Alice can apply appreciable drains to Arngrim and Dorren. Oh, and if we go this route, we don't want to make the drain human-only, because I'd expect us to run into a lot of MP-holding monsters and nonhuman casters while adventuring.
I think the second idea (mana drain of 3 per hit) is probably the worse one, given the information we currently have. It's more useful in a less dangerous and less important context, and is less assured to be significantly useful at all. On the other hand... I still prefer it, because it makes more of the game interesting. Being stronger because we're a dragon more often is more fun than being stronger because our sword has a Really Big Number. There's also the fact that we could buy like 30 mana potions instead of a 10-upgrade spear, though this spear does still have the utility of damaging Dorren's mana in the 4S fight...
How much MP does the black blade drain on hit, right now? Trence can hit mom with it to find out!
Would mana restoration amount scale with the drain amount? Would +5 mana on hit be reasonably achievable?
Are the pink lands or pits of borlia known to have a decent density of monsters with MP?