Buy the 2015 Old Farmer's Almanac!
Reporting
We're not familiar with aWe're not familiar with a disease or pest that causes black buds. Perhaps you want to dig up one and see if you have rot? Sometimes dahlias are slow to grow when adjusting to a new place and survival rate is low for the first planting. Be sure to avoid nitrogen fertilizer until plants are close to flowering. 

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?