[MadMan]

Aside from the molding process, which I know little about, I'd say regular 100% silicone you get in tubes from the hardware store should work fine. Black rtv for automotive gasket making is good for a jet black color. Although a lot of automotive 'black' rtv is actually slate gray, look among the cheaper ones for actual black. I've looked into this a bit, but never actually attempted it - apparently if you mix corn starch with silicone (look on youtube for silicone mold making) it seems to change its cured form a bit, making it perhaps a bit firmer (and I imagine stronger), but without losing flexibility. And if you use clear silicone and corn starch, it'll become opaque and you can mix in paint to change the color.

If you need a firmer, stronger material (cured silicone is still kinda squishy), you could use automotive urethane (windshield glue). It's maybe $20 a tube, and a messy pain in the ass to work with (gloves mandatory), but the finished product is a lot like black vulcanized rubber, and it sticks better than anything, really. So for a motor mount, you could 'mold' a bolt into the urethane, and I wouldn't worry about it breaking off. I actually have an old typewriter that needs new rubber feet, that I've been thinking about doing this for.

Also don't forget that rubber can be machined. If you need a structural part (not decorative), try talking to a machinist.