They can be blued with cold gun blue or hot blued.

The originals were blued like a gun but I believe later they used a black phosphate finish. It is more costly the a simple bottle of gun blue but they sell tool blacking kits or you can look on the internet for a do-it-yourself version (the main chemical is phosphoric acid).
Keeping them to long in the acid eats up the parts somewhat as you saw by leaving the parts in the degreaser.

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84 300D, 82 Volvo 244Gl Diesel