The official MW3 Content Calendar appears to have been updated and now reveals Black Box, along with Special Ops missions, Black Ice and Negotiator are arriving on April 12th for PS3 Elite Premium subscribers.
Additionally the month of May prior to this had 1 multiplayer map, 1 Spec Ops mission, and 2 classified scheduled for Xbox Elite subscribers and now it has been updated it to point out that all 4 drops are going to be multiplayer maps. This drop is going to be accessible for PS3 Elite members sometimes in June. Will update our website when a release date becomes available for the May DLC drops.
Non paid Elite subscribers on Playstation 3 should expect the very first Modern Warfare 3 content collection at some time this month. It's going to include 2 spec-ops missions, Negotiator and Black Ice, and also 4 multiplayer maps, Piazza, Liberation, Overwatch, and Black Box.