Well, looks like no one wants to touch this one - probably because there are several things that could go wrong. Here's my 2 cents...

I generally stopped buying "garden center in a pot" dahlias because of exactly what you experienced. They tend to do well for a couple of weeks and then peter out. In fact, I bought one growing about 4 feet high loaded with blooms 2 years ago and my garden 'notes' from last year reminded me that it NEVER came up the following year. (But this year it did and was I ever surprised/glad!)

I think these beauties are grown in the ground and then forced into those 1-2 gal plastic pots for shipping and sale. I cannot imagine they appreciate that so I suspect once they are re-transplanted back into the ground, they go into transplant shock from one or both transplants. IF I buy an already blooming dahlia now, I simply leave it in the plastic pot until it is done, since I know replanting it will usually end its blooming.

But why they stop blooming and turn black is the real question. I used to get black buds when I used to over water dahlias. It is possible something is eating them but usually you will see who or what is doing it to the buds or stems. My personal thought is there is something in the soil of the large barrel they did not like but only you know what soil you planted them in and that may not explain anything if the other plants you mentioned are doing fine. What I can say is my dahlias take a dim view of sharing their space with ANY other plants, in part because the other plants often use water the dahlia needs. But again, only you know how much water they get, so this is only a thought.

As to what to do? Join the majority of us that simply have to 'wait til next year'! I have learned to always give my problem dahlias 3 years and if by then they are not what you thought they were, time for shovel pruning!