Oscuro means dark. In wrapper talk it is the far end of the color scale: beyond a standard maduro, sometimes almost black. The darkness still comes from time, heat, and fermentation, plus the starting leaf, not from paint.
Because the category is informal, one factory’s oscuro is another factory’s maduro. Use the word as a relative shade. The blend underneath can be mild or full; the wrapper color does not settle that argument.
See maduro for the more common dark-wrapper term, and colorado for the reddish middle.