Unappreciated is King's emphasis on natural law and the western tradition. That emphasis gave legitimacy and moral transcendence to what could have been a merely lawless movement. One sees this attention, for example, in his Lincoln Memorial speech, Letter from Birmingham Jail (about a third of the way down), and his final speech (see third paragraph). Doubtless much of this derived from documents in the black American political tradition such as this extraordinary 1774 slave petition for freedom (it's a short document, RTWT and look at the last sentence):
That legacy is what makes this American our greatest political spokesman for natural right and therefore our most sigfnificant conservative.
This book supports the argument here.