I believe you will find James, Elizabeth, Mary (& her siblings) recorded as BLACKMAN in the 1881 census at: Dwelling: 16 Deverell St Census Place: Newington, Surrey, England Source: FHL Film 1341120PRO Ref RG11Piece 0532Folio 51Page 3
A cousin (in England) and I are still working on finding them in earlier censuses. Our interest is based on the fact that we come down from a sibling of Elizabeth Sarah PAGE, Emma Margaret (PAGE) HAYWOOD. I guess that makes us your cousins too.
Elizabeth Sarah PAGE who married James Jonathan BLACKMUR (sometimes BLACKMAN or BLACKMORE depending on the records involved) was the daughter of Edward John and Sarah (AITKEN) PAGE who married 12 July 1829 at St. Olave in Southwark.
Edward PAGE appears to be one of several children of John & Mary PAGE who were baptised in the Parish of Rotherhithe. His baptism was 6 Dec. 1801.Edward is presumed to have died before mid 1850 and his widow (as Sarah PAGE) married a James BLACKMUR (Sept. quarter of 1850). The "blended family" of PAGE and BLACKMUR children were recently found with James & Sarah (AITKEN)(PAGE) BLACKMUR in the 1851 census - that is what opened this line of research for us.
It appears that Elizabeth Sarah PAGE married her stepbrother, James Jonathan BLACKMUR in 1854.
I would be happy to exchange additional information.