The blackening of the young leaves may have been frost damage - after a mild or positively summery March, we then had several frosts and much colder temperatures.
These plants are not fussed about soil ph, so it wasn't necessary to dig in ericaceous compost. I doubt it will do any harm though, unless you've planted it directly into ericaceous compost. If its looking really poorly, I'd be inclined to dig it up and put it in a large enough pot with ordinary compost, and keep it somewhere very sheltered where it gets some sun (not a lot) and where you can keep an eye on its progress, but otherwise, you might want to just wait and see how it goes.

I think Bamboo is correct with frost. Did you have it outside much before you planted it? If it went from a nice warm nursery or shop environment to frosty cold April night it may be in shock. I agree with Bamboo on the actions too.