Hi rebeccamary,
it is possible i found a Agnes Black in the 1851 census for Buteshire,Isle of Bute aged 11 and i have the rest of the family are on the isle of Lismore in 1851 .
In the 1861 census the family are in Rothsey,Isle of Bute but no Agnes and a gap between Christina aged 22 and Alex 12 so she could have been else were.
Do you have Agnes,s birth cert,probably have to prove that here mothers name was McIntyre to proceed,
My dads god mother/aunt was Agnes ,who,s mother was Catherine Black so it is possible,
Billy.