Bearded Dragon Turned Black

Question
QUESTION: My Bearded Dragon appears healthy, is eating well, and has a normal activity level, but the skin on his/her entire body has suddenly turned almost black.  This is not a condition that comes and goes as the dragon gets hotter or colder, but rather has been consistent for a couple of days.  I would guess he/she is about 5 months old.  Is this a symptom of illness?

ANSWER: My first question would be do you have UVB lighting and if so at what strength also I would ask what the heating on the cool side and warm side are?

---------- FOLLOW-UP ----------

QUESTION: I have a UVB light, but I don't know the strength.  His habitat was already set up when I purchased him from a pet store that specializes in lizards and snakes.  They sell mostly Zilla products, and it is a long tube light if that helps any.  I think my temperature is good.  We have a 20 gallon tank and we keep it at least 90 on the hot side during the day, with a stick that he crawls up on to get closer to the light (but not too close!) to get warmer.  The cool size is about 80.  He moves around as he gets hotter and colder.  At night, we keep it at about 75-80.  He is still acting completely normal and eating well.  He eat crickets, squash, turnip greens, mustard greens, collard greens.  Thanks!

Answer
I would suggest downloading our mini-ebook from our site http://www.reptileapartment.com.  I would also suggest raising the heat slightly to about 100 degrees on the warm side which should also increase the heat on the cool side to about 85 or so.  If the light hasn't been replaced in six months its time to do so as the UVB output does decrease over time.