Any filler, especially under the delicate eye area, tend to bruise. I cannot tell from the photo what the exact dark area is. If it is hyper pigmentation and it is getting lighter as you say then it should take several more months to fade away. Please return to your treating physician for evaluation.
"Mark" under eye at 4 months injection
The picture submitted is not clear. I cannot adequately assess one dark spot under the eye. I can tell you, if you are at risk for hyperpigmentation, usually darker toned skin such as Asian, African-American, Indian or of these decents, you may have post inflammatory hyperpigmentation (PIH). To treat, use a topical gel or cream that decrease melanocyte formation. These products include hydroquinone, products such as Lytera, Obagi Clear, Glytone - Bleaching cream or Dr. Obagi's Brightonex. It may take you 6 weeks to see a result. It is important to remember to wear sunscreen because if the spot is PIH, it may reappear with sun exposure.