Scott, having read through "A History of No. 10 Squadron Royal Naval Air Service in World War I" by Mike Westrop, I can tell you that nowhere was there reference to under wing letters on any of the Black Flight planes nor do any of the photos, though mostly poor quality, show signs of the black letters. Pheon's superbly researched Triplane decal sheets don't have them either. Bear in mind that the Roden Tripe's fuselage is too short, so I don't think I'd trust that source. Maybe someone else will have something more definitive.
Cheers,
Bud