First of all, thank you for taking her in, it sounds like her life has not been good at all up to this point. Do you know how old she is? And what size of tank is she in, in either gallons or dimensions? Is it a glass tank with a mesh lid, or is it a wooden enclosure?
Dragons are Desert Reptiles, and as such they have extremely specific lighting and temperature needs, and yes, the
black beard is a sign of either discomfort, pain, or stress. Without an adequate, long UVB tube inside a matching length, flurescent tube fixture with a metal reflector inside it that is behind the bulb to reflect the UVB and UVA light from the UVb tube throughout the tank (at least 18" long, no compact or coil
UVB bulb that fits into a normal lightbulb socket/lamp is adequate for a Bearded Dragon) and a bright-white colored Basking Bulb that is the correct wattage to keep all 3 temperature zones inside the tank within the correct temperature ranges, his body cannot make any vitamin D3 on it's own, thus cannot absorb any nutrition from the food or the calcium and multivitamin powders you should be dusting his live bugs in, and cannot process or use any nutrition from his food or supplements.
He also cannot properly digest any food or supplements that he eats if his temperature zones are not within the correct ranges, which often results in a bowel impaction or obstruction, and feeding him Superworms in improper temperature zones will certainly cause a bad impaction. When is the last time he pooped?
Can you please tell us what
UVB light you have (brand, wattage or strength, and type, meaning long flurescent tube or a compact or coil bulb), what Basking bulb you have (color and wattage), and how do you have them mounted/set-up? Is your
UVb light obstructed by the mesh lid to the tank? How far away in inches is the
UVB light from his basking spot?
What type of thermometer are you using to measure his 3 temperature zones? Is it just a stick-on thermometer, or did you get a Digital Probe Thermometer or Infrared Gun? You must have at least a $10 Digital Probe Thermometer because you cannot measure his Basking Spot Temperature at all with a stick-on thermometer, they only measure Ambient (air) Temps, and they are also very, very inaccurate, often off by between 10-20 degrees when tested.
BASKING SPOT SURFACE TEMPERATURE (within the Hot Side of the tank, should be directly under both the
UVB light and the bright-white colored Basking Bulb so he gets both lights at the same time, and is where he should want to go to bask directly after eating): Needs to be between 105-110 degrees F for a baby or juvenile Dragon that is under a year old; between 100-105 degrees F for an adult dragon that is a year old and older; absolute maximum temperature anywhere inside the tank anywhere should be 110 degrees F and no hotter!.
HOT SIDE AMBIENT (AIR) TEMPERATURE (The Side of the tank that both the UVb and the bright-white colored Basking Bulb should be over top of and that contains the Basking Spot/Platform within it): Needs to be between 88-93 degrees F maximum.
COOL SIDE AMBIENT (AIR) TEMPERATURE (The side of the tank opposite the Hot Side, that has either no lights over it at all, or at most a very low-wattage, secondary bright-white colored basking bulb to keep the Cool Side Ambient Temp in the correct range if the main, higher wattage basking bulb that is over the Hot Side of the tank isn't enough to do so): Needs to be between 75-80 degrees maximum.
Please try to post some photos of your dragon and then his entire enclosure/tank and lighting set-up, and then answer the questions about his lights and temps. Chances are that his temperature ranges and/or lighting are not quite right or that his
UVb light is not adequate, and this is what is causing the lack of movement and
black beard, he may be impacted due to improper temps and lighting, etc.