The Devil Treasure Room generated will be the same as the Treasure Room that would have been generated, minus the item and room textures.
If the Treasure Room would have been a double Treasure Room, the Devil Treasure Room will also have two items. Unlike the double Treasure Room, however, both items can be taken. This also goes for Treasure Rooms on the alternate path (the blind item will still be blind).
This trinket also converts Treasure rooms on the current floor when picked up. However, this only affects Treasure Rooms that have not been visited yet.
The same goes vice-versa: dropping the trinket will convert Devil Treasure Rooms to normal ones unless they've been visited.