Geology question ....

Google tells me it's a mineral called magnetite.🤷‍♀️

That all depends on which coast you were on. The composition of the sand on a beach is determined by local rock in the area. Plus, you were in an area that's normally covered in water, so it's possible you came across another substance that's not normally visible to beach-goers. If you want, you could look up sand composition for the beaches in your area.

This is not exactly scientific, but it sounds to me like the ground gave you a giant hickey.

It "painted" your legs because it was clay. Unless sand or gravel, clay is laminar. Clay is like extremely microscopic sheets, that's why it "paints" your skin (try to do some pottery and you'll notice it) and that's why it doesn't evacuate water, which is what causes quicksands. If you have quite an important percentage of clay in a soil, and a lot of water, chances are you'll have quicksands there.

Clay is usually beige, but it can have other colors. Perhaps it was black because of the mineral, perhaps because of some organic content. If the beach itself wasn't black or at least dark, I'd say it was organic.

I'm gonna have to ask partly because of other people's answers and partly out of genuine concern. Is your leg still black? Did it wash off at all?

I can't get pass your sinking in quicksand on a beach. How did you escape?

It was probably sediment from some weathered rock that was very fine grained. I'd probably have to see it.

Was it near an estuary? If so, it was probably just decaying plant matter. I've seen this kind of thing around marsh and river type areas.