I read a book series in high school/middle school (late 90s,early 2000s) and the main character was a prince/lord/ruler of some area set in a wintery/northern area. He inherited the ability to turn into a dragon through his father. He had a few external characteristics, mainly very hard blue fingernails. This stands out to me. I believe his family's bloodline causes him to be a black dragon, almost demon like?
There are a couple other people in the story world who can turn into dragons but very few. Like the main character, the dragon they can turn into is determined by their bloodline.
Also I vaguely remember most of the story world being somewhat victorian, but the area the main character presided over was less civilized and tribal almost. The rest of the story world seems to have forgotten about dragons
Like any other young adult book he falls in love with a 100% human which causes issues. She is also stolen away by a rival ruler who is in a more civilized nation. This ruler doesn't believe in dragons and thinks he has the most modern army etc
I also think there was liberal use of the letter "K" in the book or series title but I am the most vague on this.
I can't remember if it was a particularly good series but it came to mind recently. Sorry for the crappy description!