It is probably slime mold that you are seeing.  Grab one of the blades of grass and pull it between your thumb and index finger.  The black powder (spores) will slough off and the grass blade will be clean.  Slime mold is not a serious problem.  During wet years it feed on decaying organic matter and will climb onto grass blades to sporulate. See pictures.  If you want to, you may try applying a fungicide (labeled for turf) sprayed or spread of the area. 

Bermuda likes a high nitrogen fertilizer early in the summer.  I would apply 21-0-0 at five pounds (ten cups) per 1000 square feet. Water it in with 1/2 to 1 inch of water or apply before a rain.  You should see green-up within 2 weeks or less.