Had stitches 10 days ago and directly on the scar there is like a really thin an short line of black colour directli on the cut. what does that mean?

If your stitches: were black there is a small possibility that there is thread still under the skin if the stitches were not removed properly. Or you may just have a small keloid growing. Keloid is an dark overgrowth of skin tissue usually over a scar that can develop into a large anomaly over normal skin. There are random people that get these especially darker skin folks. It is harmless.