Rockbridge Co., VA was created in 1778 from portions of  both Augusta Co., 
VA and Botetourt Co., VA (and Botetourt itself had been created from a 
portion of Augusta Co., VA earlier).  As such, the "first slaves in 
Rockbridge" had probably already been living in Augusta and/or Botetourt. 
Augusta was formed from Orange, Orange was formed from Spotsylvania, 
Spotsylvania was formed from parts of Essex, King William and King and 
Queen, etc., etc., back to the original "shires" in Virginia in the 1600s.

You might find some information in the three volumes of the large Chalkey 
document that covers Augusta Co., VA: 
http://www.rootsweb.ancestry.com/~chalkley/index.htm

Also, the 1778 (inagural) Rockbridge tithe list is here: 
http://ftp.rootsweb.ancestry.com/pub/usgenweb/va/rockbridge/census/tith1778.txt

Also, the 1783 Rockbridge personal property tax list is here (and this shows 
the number slaves, age 12 or above, in each household): 
http://ftp.rootsweb.ancestry.com/pub/usgenweb/va/rockbridge/census/tax1783.txt?cj=1&o_xid=0001027214&o_lid=0001027214

Maybe the above will help. 

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