http://en.wikipedia.org/wiki/Negro_MountainThroughout US History the name used for African Americans have changed. In Colonial American the term Negro was preferred, it basically meant someone of the Negro Race. The Term "Colored" was adopted during the Colonial period to mean someone who was of mixed race. During the time period before the Civil War, how much African American "Blood" was in an African American became a way to discriminate. During that time period the less African American "blood" a African American had, the higher social status that African American had in much of the South. Over time this gave raise to the term "Colored" for African Americans, for "Colored" implied an African American had some white ancestors and thus was better then a "Negro" who had not white ancestors (Remember I am talking about pre-Civil War American NOT today so please do NOT take offense).
During the Civil War, the North decided to raise African American units, these were called "Colored" Troops for by that time more African Americans preferred the term "Colored" to the term "Negro" (and Negro's even more hated relative, that is spelled with two gs). The term "Colored" was used till the 1950s (The National Association for the Advancement of Colored People, NAACP was founded in during the height of the use of the Term "Colored" and thus why the term is in its name). In the 1950s and accelerating in the 1960s, the term "Black" became more and more popular. Previously the term had been avoided for its implied 100% African background, something most African Americans can not claim, but with the Civil Rights Movement more and more African Americans saw a need to praise their African background not be ashamed of it, and thus the term "Black" became the preferred term.
The term "Blacks" had always been used in terms of African Americans, but only as a substitute for the Term Negro, i.e. most people divided the people of the world in the 1800s into three "Races", the Caucasians, the Orientals and the Negro Races often "simplified" to the White, Yellow and Black Races (Today's students of people tend to reject these over simplified divisions, but popular in the 1800s).
At the same time period (The 1950s and 1960s), the term "Negro" came to be viewed as the same as its cousin with two gs and many African Americans came to view both terms as the same, even through both terms had been viewed as different for centuries (In the 1860s election, an American Politician when discussing the possible election of the Southern branch of the Democratic Party being elected President, made the comment that "the American People would never elect someone President who spells Negro with two gs" showing that among even white the variation with two gs was known to be derogatory as to African Americans by 1860).
Thus by the 1960s it became the norm to referred to African Americans as "Blacks" instead of Negros or Colored persons. This has been the Norm only since the 1950s (The term Colored survived in the US Army till the last Colored units were integrated during the Korean War in the early 1950s).
Since about 2000, the term African Americans seems to be the "Correct" term for people previously referred to as "Negros", "Coloreds" and "Blacks".
Given this history it is not surprising that some people would be offended on seeing these earlier terms for African Americans still being used for things named during those time periods when the earlier terms were viewed as the "Correct" term for African Americans. Negro is one of those terms, and since it is the oldest term and the closest with the derogatory name with two gs for African Americans it is easy to see why people object when they see the term. Most people should know better, but given most Americans get their geography lessons from TV News, we should not be surprised that things named centuries ago sound like a more recent derogatory term, people will jump the the derogatory term instead of the actual wording and what it meant when the term was first used.