What is the cause of dark gum line around tooth after root canal? What options are there to correct this?

I had root canal and a porcelain (no metal) crown in Thailand 5 years ago, at first it looked fine but over time the gum line around this tooth has gotten noticeably dark to the point where I need it fixed. I spoke to a dentist and he told me I could have the crown replaced to cover the darkness, but over time the gum line will rise and the darkness will appear again. I'd like to know what the likely cause is and what options I have for permanently correcting it and how much it's likely to cost?