What insect looks like a spider but has only six legs?

If it has 6 legs it isn't a spider, it's an insect. All arachnids have 8 legs unless legs have been ripped off, in which case you'd still be able to see where the legs would be if they were still attached.

Even if you just miscounted, the description of a "small black spider" with no other details leads to thousands of answers. There is no way to tell what it is from the description.