Dental crown noticeable and feels strange. Will I lose the tooth?

It is possible that you may be getting a recurrent abscess on your crowned tooth. I would advise you see a root canal specialist for an xray to determine if it is indeed getting reinfected. The darkening around the gum is normal for a root canal tooth and porcelain fused to metal crown. They can darken as your gums recede revealing darkened root structure. You should have your dentist recheck it as there is something amiss about your description. Hope this helps.

Thank you for your question. The best judge of whether something is happening in your crown is you. So when the crown feels different, uncomfortable (some people say itchy) it usually means that the root treatment is failing and is in need of being redone.
I suggest you see a specialist in root treatment (endodontist) who your dentist should be able to refer you to or you can see directly. They will likely be able to save your tooth if you see them without delay. 
Good luck 
Thomas Dey, BDS, MSc 

The metal ion leach and the highly opaque structure of the metal within the crown is the culprit which gives the black/ dark margin around a ceramic fused to metal crown. An all ceramic metal free crown can be placed with removal of all metallic filling material inside the tooth will do the trick if there was no amalgam blue remnants on teeth. A good sub gingival crown with excellent translucent layers will correct the aesthetic issue.