There's why Night elves get mages.
Blood elves will never be Druids because of Game Balancing issues.
However, prior to Arthas decimating the people of silvermoon city (and the subsequent surviving High elves becoming blood elves), the highe lves had a ranger faction that acted sort of independantly of their main government, called the Farstriders. These high elf rangers have been known to use simple druidic magic.
In lore, Blood elves are capable of becoming a druid, but none thus far have been documented considering the race as a whole are tainted by fel magic. So, while a Blood elf druid is feasible and possible, it's just not likely, both in lore and in game.