Why are my gums grey around my 2 back teeth? Both had root canals & porcelain crowns (Photo)

My 2 back teeth both have had root canals done on them and both have porcelain crowns on them. My gums in the cracks between both teeth are dark grey and it smells bad when I floss between them no matter how much I floss and it kind of hurts to floss and I get an aching pain coming from that area even though both teeth have had root canals.