Apologies to the OP for thread hijack!
Let me try to clarify
ISO 4762 is a standard that specifices cap head bolts. In Germany (and more widely now) the standard is referred to as DIN 912. In Britain, it is BS EN 4762.
So if you search for DIN 912 bolts, you'll find allen bolts that conform to this standard.
In contrast, BS EN 4017 (equivalent to as DIN 933) covers hexagon head bolts
If you search for 4017 high tensile bolts, you can find for example:
http://astm-asme.com/din_933__iso_4017_bzp_finish_cr3_grade_88_hex_head_screw
Don't confuse strength classes with ISO doc numbers.
Back to the original contention:
1. There are high strength (more than 8.8) zinc plated fasteners available.
2. Plating can cause hydrogen embrittlement, but this is nullified by simple bake out after plating to diffuse the hydrogen out of the metal.
So it really comes down to economics / aesthetics as far as I can see.
Very high strength BZP fasteners are not all that common, black phosphate finish is more readily available, but this has relatively poor rust resistance.
As an aside, impact sockets are black because they are made of (more expensive alloy) higher ductility steel, and the Chrome plating used on standard (less expensive alloy) steel would chip / crack. And it makes them easy to identify, which is handy.
Hopefully this will prevent Engineer War