Watch the Level 1: Blues Bends online guitar lesson by Jeff McErlain from Take 5: Blues Bends
Bending is much more demanding and difficult than it may seem. I've also found slow bends to be more difficult to play in tune than quick bends. I think it leaves us sometimes too much time to think about the note and we question the pitch.
So, a lot of this is based upon ear training of course, and muscle memory. Muscle memory is extremely important and oftentimes a little easier to rely upon and then your ear in certain situations. By practicing bending, our body remembers how much pressure it takes to bend the string to the pitch that we're looking for. It's important to pay attention to the way the bend feels.