No, they don't stack.
Boosts of the same "type" don't stack, "+dmg to all" is one type, so it doesn't stack with itself but stacks with any other type such as "+dmg to human" or "+dmg to undead" etc.
If you equipped BLOD and another piece of the AFDL set, you would get (1.75 * 1.15 = 2.0125) 101.25% extra damage.
When you have multiple boosts of the same type equipped, you will get the value of the last boosted item equipped, so in your example, if you equipped the +15% dmg item last, you will actually only deal 15% extra damage, to get the 50% extra damage, you will have to unequip NSOD and re-equip it again.
Try to keep this last fact in mind, some people aren't aware of it and it reduces their damage output by quite a bit.