I had the same thing happen to me a several years ago. It started turning black the first year I planted it. I believe my soil was also getting too much water, we had just put in a north facing raised bed (dense unammended top soil) along with a new lawn and I was watering often. It was barely hanging on by a thread at the end of the season and did not reappear the following year. I think both of our problems are due to over watering. One thing you can do is dig it up and add some sand to the soil around it to help with drainage. It works quite well. I have included a link that will help out a bit I think.

Here is a link that might be useful: Check the added info at the end of the post