There are two new varieties Black Diamonds and purple
Prince. I don't know that I would 'recommend' either of them a they are not at all natural and as new varieties they've not really stood the test of time as to how they perform year after year after year in all conditions - the best that can be said is that they will have passed commercial tests to see whether the public can be persuaded to buy them!

J.Parkers are selling 'Black Prince', worryingly with no planting instructions regarding soil type given on the webpage.
Crocus are selling H.macrophylla 'Black Diamonds' with the rider "To get the best colour, you should apply hydrangea colourant at regular intervals, but even when grown in normal or alkaline soils the flower colour will revert to a good shade of pinkish red."

Quite honestly if you want a hydrangea that will stay true no matter what soil type it is in, you need to choose a white variety - try H. macrophylla 'Soeur Therese'. Some of the white varieties with become pink tinged as the flowers fade, but they won't fade to blue.

I have a dark red that retains its colour if that's any encouragement, but as it was here when we came I don't know the variety. I put some lime round it one year to encourage the red colour and astonishingly it developed blue tones, which is the opposite of what it should have done so now i I leave it alone and its fine.

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