Really cool a friend gave you some cards to begin with, those people are the best. About your questions:
1) What sort of competitive decks can be made with B/U ? If none, what sort of competitive decks have in the past been made with a combo of these colours or mono of either B or U (I know about Blue Bull$H**, Reanimator, Doomsday...um probably more but feel free to refresh my memory). Yes, yes I know that "mono-black is dead" and so is BB..... And if you are going to list successful decks of either or both colours from the past, please briefly explain why they are no longer viable.
The best way to find these decks, is to go to the deck database of starcitygames, which you can find here:
http://sales.starcitygames.com/deckdatabase/index.php?exp=vinYou can chose which colours you want to search in. I think the best decks will be: TPS, Hulk Smash and Doomsday. The decks are sorted by tournament results, so you'll only see good decks.
2) What exactly is a fish deck and how does it work? I have read several articles, but am still a little hazy....
A Fish deck is a deck that packs a lot of hate and small creatures to provide the kill. Hate is mostly provided by cards like chalice of the void, null rod, wastelands. There are a lot of different strategies a fish deck can follow, so there are also a lot of different build of fish (U/R, U/W, with or without aether vial), if you want to play fish, again go to the deck database and find a build you like.
3) What makes Oath such a powerful deck? I have a basic understanding of how the core card works, but what makes it so fast/effective?
Oath is a really powerfull deck because it's really explosive. It runs a lot of control cards (FoW, drains, etc) and it can have a active oath on turn 1 (forbidden orchard, Mox, Oath of Druids with FoW backup is a something very hard to recover from).
a) Why does Oath see so much more play than reanimator? Both seem to share the common goal of bringing out a beatstick. Is it simply because Oath is faster (in that you must first dump creatures with rean) and you may reuse its effect?
Oath can be played as a control deck, so it doesn't really have to win ASAP. Also there is a lot of graveyard hate in the format, any of it will shut down reanimator.
Can't answer B, I haven't seen his list.
5) I have Tendrils of Agony and see it obvious potential, and have also read that several decks have been based around it. However, I can't seem to find the most effective combo/deck (keeping in mind that I wish to use blue/black or mono-black, as I don't think mono-blue would do much for a tendrils deck ) to abuse it. Any help is appreciated. And could Life Drains be built into a Tendril deck, making an effective "black burn" deck (also very appealing to me)?
The best tendrils deck for me, is TPS. Meandeck Tendrils is possibly the most powerfull Deck, but it is really hard to play it. TPS has protection in the form of FoW and Duress. Life Drain is not a card I would play in Vintage (or any other format), first of all: Consume Spirit is a lot better, second: if you have enough mana for a decent (read big/lethal) life drain, there are better ways to win the game, even with less mana.
I hope this covers most of your questions, English is not my native language, so I hope you can understand What I tyred to explain.
Have Fun.