BoP equipment that you cannot use, can either be disenchanted or sold to a NPC.
BoP recipes can not be disenchanted and thus have to be sold to NPCs. You could however decide to learn the appropriate profession later on and use the recipe then.
Generally, when a dungeon/raid party has an enchanter, people will abandon the 'Greed' roll for BoP items and let the enchanter perform a greed roll; this ensures that people who actually 'Need' it can still get it, without having to compete with the enchanter, whose need is lower. Depending on how the party is organized, the members may also
/roll in order to determine who will get the enchanting materials (although I have never actually seen such a party).
Of course, this is assuming the enchanter has made him/herself known to the other party members.
Blue items are not necessarily BoP and neither are Purple items, therefore there is no chance for an uncommon or rarer item to be BoE instead of BoP. They are either BoP or BoE and that's that.