O-10, Regina Carter's main influence (by her own admission) was the great Stephane Grapelli, and probably the reason for your astute observation about her tone. In addition to that it is possible that her violin is slightly larger than typical. Not all violins, while still considered "full size", are of the exact same size, and from pictures and videos it appears that her violin may be slightly larger than typical and that would be a contributor to the warm sound that you hear.
Regina Carter is a fine player and I agree with your observation that she has really blossomed. Her playing has not always impressed me at a level commensurate with the rep. I am very impressed with her playing, but have to temper my enthusiasm when considering the level that I think she is striving for and not quite reaching yet in a consistent manner. She plays with a really great feel and jazz sensibility and has obviously done her homework, but there are some technical issues that, for me, distract from full enjoyment compared to, for instance, Grapelli's lofty standard. Pitch is something that great players use and manipulate as a way to add color to the tone and to project a certain attitude; by playing slightly under the pitch a certain "bluesy" attitude can be projected. However, sometimes that is simply the way the player hears things all the time and/or is the fault of a technical deficiency and there is a constant sense that she is always reaching for the pitch and not quite getting there (flat); for me, it's a distraction that isn't necessarily a deal breaker if the music is really happening in other ways. I know some will think I am nitpicking and my comment may seem unfair, but I think there is a bit of hype going on here for a variety of reasons. None of this should be surprising; I have never heard or read a qualifying comment about Grapelli's playing "having blossomed". She is a really good player, but the bottom line for me is that I don't hear enough "there there" to want to go out of my way to listen to what she has to say musically; and, if simply violin playing is what I want to hear I would prefer Grapelli (admittedly more steeped in a "hot jazz" vibe):
https://m.youtube.com/watch?v=Ih_62WMfWEchttps://m.youtube.com/watch?v=JnhgapeBWuEAnd for someone who is still with us:
https://m.youtube.com/watch?v=r4kvzWLSDT4
