What’s an ideal weight for a BBL? I'm 5'10", 280 lbs

The data suggests that a Body Mass Index above 30 will place you at increased risk for complications during and after your elective procedure. These complications include infection, blood clots, delayed healing, loss of transplanted fat, and several others. While BMI is just a guide, there should not be a Board Certified Plastic Surgeon willing to operate on you at such an elevated weight. You should be closer to 220lbs. If you are having difficulty losing weight, seek assistance from your primary doctor and you may even wish to consult with a bariatric surgeon to discuss surgical weight loss surgery.Watch the video in the link to learn more about BMI and why it matters.

A good candidate has enough fat available for transfer in the procedure. The amount of fat that can be transferred in a BBL depends on how much fat the patient has to harvest and the amount of space there is to move the fat into. It is recommended for the patient to have a BMI of 30 or less. If you are too thin, a Brazilian Butt Lift may not be possible.

Optimal candidates for surgery should have reached a healthy stable weight preferably with a BMI < 30. My recommendation is for pre-op weight loss. This will lower your overall surgical risk profile and likelihood of complications. 
With regards to your specific question, there is no quick fix. Cosmetic surgery is not a means for weight loss. Traditional diet and exercise are the first line. For those patients with a BMI of 35 (you are currently at BMI of 36) and associated comorbidites (diabetes, hypertension etc) you may qualify for insurance coverage of bariatric surgery (surgical weight loss).
I would estimate a weight loss necessity of nearly 70 lbs (in order to dip below BMI of 30).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).