Probably a maid-servant rather than a slave.

Slavery was certainly not unknown in Nova Scotia, but according to Raddall (1971, p. 121):
"After 1792, through the influence of Chief Justice Strange, a stout emancipationist, the owners of runaway Negroes found it difficult and finally impossible to recover their 'property' through the courts. This rang the knell of slavery in Nova Scotia."
The identity of Mrs. Beamish's black girl is not known.