petunia problem

I have a breeze block wall (the blocks with the two holes) in my garden which I planted up with petunias.
Although I put them in plenty of compost and watered regularly they seem to be dying off. The most noticeable feature is the foliage which is turning black & withering although quite a few blooms remain.
I thought maybe the blocks were drawing all the water away from the plants so I gave extra attention to that but there's no improvement.
I would greatly appreciate any advice.
Thanks in advance.