Q. Why do my dahlia buds come out black?
So sorry that you missed this season's dahlias.
Yours may have had botrytis blight which causes a dark, sometimes fuzzy, appearance to the bud. It is a fungal disease that is common for dahlias in cloudy, wet weather. You can reduce incidence by giving plants space for air circulation and keeping your dahlias in a very sunny spot in the garden where moisture will burn off all day.
Only water these plants in the morning and at ground level so that your splashing doesn't add to moist conditions.
Courtesy of NYBG Plant Information Service