Just removed big bed from a spare bedroom and this is what I found (and I'm worried)
 

It seems to be mold. I'm worried and I don't really know how bad it is.

http://i1101.photobucket.com/albums/...15/photo-1.jpg

http://i1101.photobucket.com/albums/...515/photo3.jpg

http://i1101.photobucket.com/albums/...515/photo2.jpg

http://i1101.photobucket.com/albums/...515/photo1.jpg

You may get lucky and it's just from the house being to humid and there was no air cirulation behind the bed.
Mist it down with bleach and water, clean it off and keep an eye on it.

When I see damage like that along an outside wall it's often caused ice dams on the roof.

Do these walls face towards the outside of your home? If so you may find you have no insulation in that wall as I found in a house I owned that was built in the 60's. I kept finding that the walls were sweating and that mildew would form only during the winter months.

I don't understand how code would not require insulation on outside facing walls, but I knew of many subdivisions in my town that had none.

Yeah on exterior wall.

I think I have blown insulation all the way around the house (there are plugs in siding).

House is about 90 years old.

What really scares me is what's behind the walls. I worry that there's severe mold and rot to the studs.

Do you think the moisture is from the inside?

It may be just surface mold. Bleach & water it like joecaption said and see if that takes care of it. I did that at my mother's house and it was fine. She was running a humidifier 24/7 in the room where she had the problem and the surface mold had started in a walk-in closet where the clothes were packed in tight. We got rid of the old clothes, aired it out/cleaned it up, reduced the humidifier use, and it's been fine ever since.

The trouble with trying to blow insulation in an outside wall in an old house is you can not see what your doing unless you have X ray vision.
If that wall happens to be an outside corner there could be a diaginal brace built into the wall blocking the insulation from reaching the bottom.

Well, after spraying the areas with a mixture of water and bleach, it seems to have completely gone away. Hopefully I'm lucky and it was only INSIDE the house and not inside the wall. I still have some cracks in that wall that will need to be addressed. I will wait to put up new drywall until after I have taken care of some structural/foundation issues with the house.

joecaption: what do you think of closed cell sprayfoam in exterior walls when the wall is completely open (as in drywall replacement)? I think I will probably be redoing that drywall BEFORE I reside the house (another excellent opportunity to assess and improve insulation).