Yeah, the dark material on the reverse looks like some kind of glue residue. It potentially lowers the value of the coin. You might be able to remove it by soaking in acetone but if it does come off the underlying coin surface may have a different color than the unaffected areas.
Mintage for the 1870 was over a million pieces so they aren't that hard to find. For what you paid you should expect a problem-free VF coin, without glue or other potential problems. IMO you could do better.