Is the sagging skin of the gluteal fold post black due to weight loss or a BBL done incorrectly?

Based purely upon your history, it sounds as though the aging process and weight loss are the primary reasons for your fold of skin at the buttock-thigh junction (a.k.a. infragluteal fold).  Depending upon its severity, fat grafting may help some but typically a lower buttock tuck (a.k.a. banana-roll plasty) is necessary to best eliminate and redefine this area. Glad to be of help

Hello, thank you for your question. It is hard to tell with a photos, I recommend you to schedule an email, Skype or in-person consultation with a board certified plastic surgeon for proper evaluation.

Dear Sunshine0910,
I understand your concern. However, without a proper assessment it would be difficult to determine what is wrong. It is best that you visit your plastic surgeon for further assessment. Only after thorough examination you can get proper recommendations and advice.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Dear sunshine,Photos would have helped answer your question more accurately.  From your history alone, it sounds like the weight loss and aging most probably contributed to the appearance you now complain.  I do not feel the BBL 7 years ago led to your present predicament.Dr T

It is hard to answer your question without photos or a consultation. This can likely be resolved using high definition VASER liposuction or a second BBL, but you should consult a board certified plastic surgeon. The definitive correction for liposuction deformities requires the advanced technology of ultrasound assisted VASER liposuction. As an official trainer of VASER liposuction, I encounter suboptimal liposuction outcome referrals weekly. VASER liposuction has been the only modality that has consistently corrected liposuction caused deformities. VASER liposuction ensures a smooth contour regardless of severity of contour irregularities by virtue of being able to remove any and all remaining fat comprehensively and uniformly. Finally, when skin redundancy irregularities are observed, Renuvion technology can be simultaneously applied to tighten your skin to optimize your overall contour outcome. A Skype or FaceTime consultation with a VASER specialist will help you get an accurate recommendation and pricing. 
Respectfully, 
Arian Mowlavi MD, FACS
Realself 100 Hall of Fame Inductee