I would like to add some information that might be useful to us all. I've just come across a family tree that includes some of these names I think you might be looking for.
Harcourt Blackett is a distant cousin of mine through my grandfather and he does indeed have a lot of information that he's inherited. I'm in the process of trying to get all that information from him. What I have so far is as follows: *note that some of you might be able to fill in any gaps or additions where you see fit* I'm related through Isaiah Blackett he is my great great grand father.
Blackett Family of Scotia St. Andrew Barbados
Robert Edward Blackett b. 1839 (parent Edward Blackett)
Married 1866 St. Andrew Parish Church
Catherine Jane Grace Marshall b.1840 (parent: Bryan Marshall)
James Edward Blackett b. April 2 1867 (married Rosina Worrell Aug 22 1896)
Eleanor Keturah Blackett b.Aug 7 1869
Isaiah (Isaac) Blackett b. May 28 1871 (married Annie St. Clair King)
Margaret Jane Blackett b. Oct 17 1873
Theresa Pricilla Jane Blackett b. Dec 6 1875
Fitzwilliam Augustus Blackett b.Aug 19 1878 (left Barbados and went to Panama not of sure what happened to him)
Joseph Nathaniel Blackett b. Sept 30 1880
Ernest Mottley Blacket b. Oct 18 1882 (married Glencora Downes)
I also have a few records of those who had children if you are interested in seeing those as well.
I hope this helps some of you if not all of you.
I can be reached at (firstname.lastname@example.org