So it turns out there are at least two different types of secret rooms. I just posted this on HoS' walkthrough, but I'll repost it here in its entirety for anyone who doesn't check that page regularly:
________________________________________
____________
Alright, so to find the secret room by bomb, you really need to just plant bombs in the middle of walls that don't have anything known on the other side.
Also, there are actually TWO secret rooms available. First of all, it's relevant to know that my game was entirely demonic: I started out with Vampirism, which gave me a stupid grin and hair like Dracula's, but more importantly made it so that every some-number of kills, I healed a full heart of health. After beating the first boss, I got Breakfast, which gave me a permanent health upgrade; this is relevant (I think).
In the second level I got a 10 bomb upgrade from a random chest, which gave me horns and a fuse next to my head. I later acquired the tarot deck, started with XX Judgment and played it, and got the dude looking for coins. Gave him two, got XVII The Moon. Played it, got to the main secret room. Picked up my 1up mushroom and left, then went back to the begging dude and gave him two more coins, and he gave me a teleporter.
After I'd cleaned out the rest of the level I used the teleporter, and rather than being taken to a random room in the level, as seems to be the usual habit, I was taken to a second secret room, in its own level - the level map showed just a black map with a white question mark in it. It was grey and decorated with skulls, just like the combat room on level 2, and had the two fires just like every other free loot room. There was a door to the left, which I should have taken to see what was through it, but didn't. In it I found two items, both of which were available for the cost of two permanent heart levels: "The Mark" which was a 666 arranged in a circle, which gave me damage up, and a black cat, which gave me 9 extra lives. After getting both of them I was down to 1 permanent heart level, and was then taken back to the starting room of the level.
So I ended up having gone through two secret rooms, and having 1 heart, 10 lives, and 12 bombs. However, my level 2 boss was Gurdy, who is a fucking asshole. Turns out when you die on a boss and re-enter the room, his health resets, so even though I got him down to 1/3 health at one point by expending almost all of my bombs on him, I was unable to beat him. Not that I needed to, though, since I already have that medal.
So there you have it; two secret rooms. I do not know whether you have to be "evil" to get the second one.