Last week, I opened several thousands Bellem's Boxes (2,000 boxes per purchase; made several purchases over a few days) and got 142 keys. The following is a pie chart for the distribution of key types.
Out of 142 keys, there are only 4 keys for Barrow de la Muerte, also known as Room of the Dead and Sea Cave of the Revenant (see here). The Key [Prison de Joaquin, Corridor of Assize] is needed for Selva's recruitment quest, and the respawn timer is 2 hours (see here).
The other 3 keys summoned the dungeon bosses (channel 1 only) with a reported level range of 41-66 (absolute upper/lower limit is unknown; see here). I tested with AQM Hollow keys, and found the reuse timer for keys to be about 10 minutes. All 5 types of keys also drop from mini-bosses in the respective dungeons.
For other analysis of Bellem's Box drops, see Journeys Through GE: Bellem's Boxes and Fontessa's Bellem's Compendium.