We can start by going to the original record of the marriage at the Ardennes archives, which is likely to have more info than on familysearch.
http://archives.cg08.fr/arkotheque/etat_civil/index.phppick Givet as the commune, note that the year range you want is "VII-1817 ", VII refers to the French Republican Calendar.
The marriage record is on pages 360/361 of that image set (can't link directly).
3rd Jan 1809; marriage of Samuel Blackmore, prisoner of war, English, midshipman of the Royal Navy, native of Exmouth (possib.), England, not previously married, born 10 November 1776, son of the late William Blackmore and Sarah Elson, to Margueritte Benfield, English, native of Exmouth, not previously married, born 2 October 1775, daughter of the late William Benfield and Elizabeth Clarke.
There is also a son Samuel Joseph registered to them in the same year.
Also I found a reference to a daughter b. Verdun online; checking, there are several children registered to them in Verdun
http://archives.meuse.fr/Sarah, daughter of Samuel Blackmore, midshipman of the Royal Navy aged 30, English prisoner of war, and Marguerite Banfield, his wife, b. 29 December 1807
Samuel, son of Samuel Blackmore (aged 35) and Marguerite Benfield, b. 5 August 1810 d. 28 August 1810 aged 3 weeks
Marguerite, daughter of Samuel Blackmore (aged 36) and Marguerite Banfield his wife, b. 20 Oct 1811
Jeannie
James, son of Samuel Blackmore (aged about 39) and Marguerite Benfield his wife, b. 9 Nov 1809