First off, sorry for your losses, I can relate as lost a hive or two up north long time ago. You say that the brood comb is black,/ Is that because the bees are head first inside the cells? Or is there something else not described. If is bees headfirst inside the cells, then I can say with certiany they starved out. no honey reserves that they either could reach or consumed all within reach. The bees can't travel far in the winter monthes to secure honey, thats why they try to position the winter cluster very close to substancial stores to avoid excessive travelling away from the warmth of the cluster, is also why they prepare the brood chamber with stores in the top and corners with honey and pollen, remember they are always raising brood but in a very limited area durning the winter. If what I have described is not the case, please give more information about whats happening. Bees need atleast 60 pounds of honey going into the winter thats a minimum.
as Barry suggest you need to provide a bit more information.... although given the timing (some things do seem to operate on a clock of sorts) starvation is a good guess.
Bees need atleast 60 pounds of honey going into the winter thats a minimum.
likely not a bad guess* but such things as length of winter and size of hive would be major variable in calculating necessary stores. actually the one or two research papers I have read in regards to stores and overwintering the essential food resources for a hive to make it to spring time was calculated to be something between 20 and 22 kilograms of honey. this number seems to have only been the essential requirements to get a hive thru till the spring time, so stores related to spring brood rearing (which can be quite variable and considerable) were not in this particular study.