How to maintain an old brick wall
Soot sounds good to me . To have localized areas outside the flue (where the smoke goes up) is bad news. The loose mortar needs to be removed and the mortar repointed (new mortar pressed into the cracks and the joints filled). 

Once you have the crumbling mortar removed and repaired with fresh mortar you should not have any more soot getting out. 
 Trisodium phosphate is a good cleaner that will take the soot off with only a little elbow grease. Let us know how it turns out.