Jacob Blok was a son of David Hartog Blok and Betje de Leeuw. He married as a 27 year old 2nd Lieutenant on 4 February in Den Haag Mietje Zon, a daughter of Abraham Jacob Zon and Henriette Cohen. The couple had a son, Alfred David Henri who was born in Batavia (Dutch Indies). The marriage ended by divorce which was registered in the Civil Registry of Den Haag on 6 March 1902. On 2 December 1903 Jacob Blok remarried in Amsterdam to Abigaël Annette Pinto, daughter of  Louis Pinto and Annette Abigaël Roozeboom..

City Archive of Amsterdam, archive card of Jacob Blok and index Marriaged of the Municipal Archive of Den Haag. (www.digitale stamboom Den Haag)and website www.wiewaswie.nl.