Stained in black spots & mildewed means it is serious black mold. If it is greater than 10 square inches, get a mold inspector to look at it and he will recommend ways to remove it.
And yes black mold is serious stuff, it can lead to mold poisoning & Asthma, Nausea & stuff like that. You could also try to remove it yourself using these tips
http://www.blackmoldremovalsupport.c...mold-tips.html
I personally found them useful.