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. A 4NT bid to ask for aces (or in RKC, keycards) 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 A 4NT bid to ask for aces (or in RKC, keycards) Blackwood won't tell you if you belong in slam, then don't use it.

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