accidentally completed the main quest on Bloodmoon

Only you could "accidentally" complete an expansion pack...

Have to wait until I can get a copy of Tribunal...
Plenty of 2nd hand copies on ebay and amazon, so you shouldn't have to wait too long.
I don't surpose once I have done THAT that I can transfer my existing character to Oblivion? I hate to say goodbye to my humble level 70 high elf mage....
Unfortunately not. Each Elder Scrolls game starts from scratch, with completely different character possibilities (different stats, attributes, spell effects etc.) so there's no way to port your character.
Level 70 is a good level though to reach - and by the time you've finished tribunal you can either retire the character or start adding mods to extend the possibilities for both character advancement and quests.
Or you could even start a new game of Morrowind with a new character, roleplaying and choosing completely different skillsets and quests than you chose before - some questlines will have locked you out of others as a result of your actions, so it's not possible to see and do everything the game has to offer with one character.
Having said that, once you've finished Tribunal (which is more linear than the game you'll have experienced so far) I'd suggest starting afresh with Oblivion - a whole new game to explore.