Is it true that the given name "Susan" is particularly unpopular among
people of African heritage, and if so, why? Is it more associated with
the abuses of slavery than other names?

Nothing definitive, but it might be included in this book by Jonathon Green:
http://tinyurl.com/algv  (redirect to Amazon)
Words Apart: The Language of Prejudice

Hello tempem-ga,

Although Susan does not to appear to be a very popular name among
people of African heritage, I don't believe it is "particularly"
unpopular.

I know 1 or 2 African American women named Susan.

Here's one prominent African-American woman named Susan:

TheHistoryMakers.com, Susan Taylor
http://www.thehistorymakers.com/biography/biography.asp?bioindex=74&category=mediaMakers

Essence.com
http://www.essence.com/essence/aboutus/companynews/0,16109,265968,00.html

Googlenut

I have never heard that the name "Susan" has any particular
connotation among African-Americans.

Here is a document that lists names from the records of a Civil
War-era African-American cemetery. More than 1000 names appear on the
page. There are 3 entries for "Susanna" and 5 for "Susan," so it
doesn't seem to have been an overwhelmingly popular name for slaves.

http://www.freedmenscemetery.org/Ffxbirth.PDF

Here is one possible suggestion that Susan was a common name given to
girls or women who were slaves.  I tend to think that Booker T.
Washington was just using the name "Susan" as an illustration; but
perhaps he did so because Susan was a common name.

"Booker T. Washington: Up From Slavery: An Autobiography - Chapter 2.
Boyhood Days"
The Literature Page
http://www.literaturepage.com/read/upfromslavery-22.html