Diabetes does hamper the ability to heal, so bruises or cuts hang around longer.

But after a year the (bruise) should be gone. So as noted above, either just a dark patch or something possibly unrelated to your diabetes.

If you google diabetes, dark skin, several possibilities come up. I don't know if you have done this. Because of changes to blood vessels, it is possible to have darker areas of skin on your body. This may be what you have.

I had bruise from my diabetic on my leg it took the time but it healed the level spot but that was okay you just have to be careful