It might not have been at full happiness. try using special methods that will make your Pokemon happy like the barber on join avenue, or get berries from the girl who sells berries on the route left of Nimassa city, or use it in battle a lot. When you do all that stuff call bianca and ask her about friendship and if she says that your both so close and it makes her happy than that means it should evolve into umbreon unless it
s daytime which is usually before 8 PM. If she dosnt say that then just keep trying. Hope this helps!