Darkened Belly Button After Tummy Tuck

In all likelihood, this is normal but it should be evaluated.
As mentioned it could be a keloid, hypertorphic scar, hernia, post-inflammatory hyperpigmentation, eschar, etc.
Your surgeon is the best person to make this distinction.
If it is thickened scar, silicone sheets or gel along with sun blocks may be beneficial.
Skin bleaching agents may also help but generally are not useful.
Avoid Mederma, it has done little for my my patients.

Hi-
Based only on your description, it's difficult to say exactly what might be going on. It sounds like you may have formed an unfavorable scar- your best bet is to visit your surgeon. At 6 months, your scar is technically still undergoing maturation changes for another 6 months to a year, and so modulation of the scar is still possible. Your surgeon will be able to advise you on their preferred treatment options, understanding exactly what was done in your surgery.
Many good options exist, including silicone tape, Scarguard gel, and topical or injected steroids, depending on your surgeon's preference, your preference, and the exact appearance of your scar.
I hope that helps, and Happy Holidays!