I found a list of the Secret Achievements for The Darkness in case you are interested. The list is below and here's the link

http://www.achieve360points.com/guides/thedarkness_AAG_P4

Secret Achievements
42. Beginnings (25 pts)
Complete the first part of the game.
43. Into the Dark (50 pts)
Complete the second part of the game.
44. Crazy For You (25 pts)
Acquire Black Hole.
45. Hills (25 pts)
Acquire Darkness Guns.
46. No Man's Land (25 pts)
Acquire Demon Arm.
47. Happy Birthday (25 pts)
Acquire Creeping Dark.

These unlock during the regular storyline, some seem to only unlock after hitting a checkpoint (by dying or continuing far enough), others unlock right away. If it doesn't unlock right away, don't worry about it. Next time you see the save icon, it should unlock then.

48. Darkling Master (15 pts)
Acquire all Darkling types.

When you eat certain peoples' hearts throughout the game, they'll unlock new darkling types. There are 4 in all, and you should unlock this naturally just by going through the game since they are all storyline related.

49. One With the Dark (25 pts)
Achieve maximum Darkness level.

You level up your Darkness power by eating hearts. This is unlocked at 180 hearts eaten.

50. Keeper of Secrets (10 pts)
Somone's waiting by the phone.

Dial any 19 of the secret phone numbers. These are found on walls, posters, and generally all over the game. They do not unlock a collectible when dialed. See page 8 for a list of all the numbers.

Thanks. I was wondering what a few of them were, as I have some of these achievements currently.