The Black Dahlias

We bought a dahlia plant about a month ago (middle of the photo), and while it had two big red flowers on it at the time, not another single bud has opened since we planted it. There are slugs in our garden which we sometimes find nibbling flowers, including the ends of dahlia buds - some of them go black as if they are rotten, and you can just make out the dark red inside. We've started removing them now. The other young buds just refuse to do anything, almost as if they are scared to open (I don't really blame them, there are a lot of slugs).

Is there anything we can do? The big pink thing (Cosmos Sonata, I think) took a few weeks to settle in but is now blooming well.

 

Its early-ish for Dahlias to be flowering. The plant in the garden centre was probably forced, so that it was flowering when you bought it. I expect it will start flowering in the next few weeks, just keep it fed and watered.

How tall is it? Bit hard to tell from your photo, and different varieties flower at different heights, but unless it is a bedding dahlia then its likely to need to be knee-high at least before its big enough to be flowering; Dahlias start to come into their own, flower more profusely, as the days shorten - hence why it is "early-ish" for them to be flowering now, particularly if you only planted it a few weeks ago.