In the frustrations around Alexander McIntyre, I found an another Alexander, an Alexander McPherson who I think could be brother to Sarah.
This Alexander, a mariner, only appears in one census that I can see (haven't looked for 1841) before his death in 1881 in Oban. He married an Ann McRae (3 March 1846 in Strath) and this is the family in 1851:
Alexander Mc Pherson 30 fisherman b. Glasgow
Ann Mc Pherson 29 b. Lochalsh, Ross
Catharine Mc Pherson 10 Months b. Lochalsh, Ross
www.familysearch.org/ark:/61903/1:1:XBSC-L6JJanet Mc Rae 18 sister in law b. Strath
Address: Dunan, Strath, Inverness
Additional confirmation with Alexander showing Glasgow as his birth place that the entry you have for Sarah is likely the correct one.
Two more children showing in later censuses, Norman born pre 1855? and Flora
www.familysearch.org/ark:/61903/1:1:XYPV-LBHCatherine, daughter, reported his death and looks to have put Sarah Black for his mother rather than Flora.
Monica