Once a partnership is in the slam zone, either player might use Blackwood.

"Regular" (or "Plain") Blackwood:

4NT asks for aces and then:

5 = 0 or 4 aces
5 = 1 ace
5 = 2 aces
5 = 3 aces

The asker can then bid 5NT to ask for kings, with the same schedule of replies, one level higher.

This convention has been around since the 1940's. It is a most helpful convention, but is often misused and abused. Blackwood should not be used as a crutch to determine if there is a slam, but more to make sure that you don't reach a slam off 2 aces (nor a grand slam off 1 ace).

For examples of the typical misuse of Blackwood, please read my article on slam bidding. A good rule of thumb:

If the answer to Blackwood won't tell you if you belong in slam, then don't use it.

For experienced players, see also RKC (Roman Keycard Blackwood).