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You can source the standard black Prospex silicone strap that they fit to the turtle, etc. The steel keeper and buckle are maybe a bit shiny for the style though.
Edit: actually the “ninja turtle” ( https://www.seikowatches.com/global-en/products/prospex/SRPC49K1 ) has a black buckle and keeper version. Wonder if you could find the part number and order it. Bet it would be prohibitively expensive though.
I want to find a black replacement that is the equivalent quality of the factory green strap. The original silicone strap is just so soft and not plasticky at all.