Negro is no more a slur than is Caucasian.
Caucasian isn't exactly free of racist subtext, itself.
From Wikipedia:
Caucasian raceThe term Caucasian race(or Caucasoid, sometimes also Europid, or Europoid[1]) denotes the race or phenotypes of some or all of the indigenous human populations of Europe, North Africa, the Horn of Africa, West Asia, Central Asia, and South Asia.[2][3]
The classification has been used in scientific racism, a concept that asserts the superiority of one race over another.[citation needed]
In common use in American English, the term "Caucasian" (rarely supplemented with the word "race") is sometimes restricted to Europeans and other lighter-skinned populations within these areas, and may be considered equivalent to the varying definitions of white people. The term continues to be widely used in many scientific and general contexts, usually with its more restricted sense of "white", specifically White American in a US context.
Origin of the conceptThe Georgian skull Blumenbach discovered in 1795 to hypothesize origination of Europeans from the Caucasus.
The concept of a Caucasian race or Varietas Caucasia was developed around 1800 by Johann Friedrich Blumenbach, a German scientist and early anthropologist. Blumenbach named it after the peoples of the Caucasus (from the Caucasus region), whom he considered to be the archetype for the grouping. He based his classification of the Caucasian race primarily on craniology. Blumenbach wrote:
"
Caucasian variety—I have taken the name of this variety from Mount Caucasus, both because its neighborhood, and especially its southern slope, produces the most beautiful race of men, I mean the Georgian; and because all physiological reasons converge to this, that in that region, if anywhere, it seems we ought with the greatest probability to place the autochthones (birth place) of mankind."