You’ve never heard it probably because it’s so completely out of style, and universally regarded as gauche.
From the late 19th to the mid-20th centuries, it was common to hear well-meaning white people call some particular black person “articulate.” Eventually, it dawned on people that it was condescending and insulting.
Complimenting People of Color (PoC) who are arti...er...well-spoke...er...good talkers can be dicey. Fortunately, Harry Reid - an enlightened liberal and certified "Friend of People of Color" - has provided us with an actual example that we can use to guide us, should we ever wish to compliment a PoC for their speechifying abilities.
Of course, each situation will be unique, but this illustration can provide you with a valuable guidepost, if you ever need it: Simply say something like "Say there, my fine fellow, you appear to have the ability to "speak with no negro dialect," unless you want one." Using that magical phrase will let the PoC know that a.) you're a Friend, and b.) you recognize the PoC's bilingual capabilities. With this simple act you will have placed the first timber to the Bridge of Trust that will cross the Great Racial Divide and lead to a brighter tomorrow.*
*Full disclosure: I haven't actually tried this, but I'm pretty sure it just might work.