I’m interested in getting a BBL. What are the risks for pulmonary fat embolism, blood clots, stroke and death?

Most serious complication is fat embolism. Small amount of fat is injected into damaged blood vessels during fat transfer; it travels in the blood stream to patient lungs, heart, brain, kidney and creates significant problem including death. Lethal outcome was observed in 1 out of 3000-4000 cases of large fat transfer. In all recorded cases where autopsy was done, it was found that fat was injected into muscle or between muscles where mayor blood vessels are located. For that reason most board certified plastic surgeons will now inject fat grafts only into space between muscles and skin of the buttocks. This space is smaller by means of volume, it has relatively limited expansion but seems to be much safer. Because of limited space for fat grafts patient may not have as significant augmentation as it would be possible using all layers of buttocks muscles, between muscles and subcutaneous tissue (space between muscles and skin). Same patients who desire large augmentation may not like these limits but it is in the interest of their own safety.
Tromboembolism (legs or lungs) is most frequently seen as a complication of tummy tuck but can be seen in 3-4% of patients undergoing liposuction/fat transfer in particular those who have multiple  risks like obesity, birth control, smoking, diabetes, multiple pregnancies, multiple surgeries, history of tromboembolism or family trait. We routinely place these patients on postoperative anticoagulant therapy. However, this condition can happen even 2-3 months after surgery! Hope this helps.

hello thank you for your question and provided information as well .. based on your question, any surgery is smaller or bigger has its percentage of risk, but if the patient is well prepared, all the prevention protocol is carried out, it does not have to have risk

Fat embolus is a very serious and known complication of fat transfer (which has been recently spotlighted in the media). While it is true, injection of donor fat into the infra-muscular plane (within the muscle) increases fat viability, this does come with a significant increase in risk. There are large vessels deep to and within the muscle which can be inadvertently injected while grafting leading to a fat embolus. The consequences of such a complication are very serious and life threatening.
BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund has convened a task force as well. The hope is that we will have more concrete guidelines for addressing these very serious complications.
That being said, the safest way to avoid fat emboli is to stay in the subcutaneous plane (at the expense of fat viability) and avoid the deeper muscular plane. If grafting of the deep plane is planned, surgeons should consider blunt cannulas and aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed. While this will likely impact graft viability negatively, it will also lower the risk of a serious complication.
According to AAASF data the risk of death from BBL will range from 1:3000 (subcutaneous grafting should reduce this risk relative to intramuscular grafting). By comparison the risk of fatal complication from abdominoplasty is 1:13,000. It is unclear how the relative risk falls with subcutaneous injection vs. intramuscular injection (however, in all likelihood, it is less).
No result is worth risking one's life for. I find that the patients who focus not on the volumization but rather the contouring benefits of the liposuction are happier than those whose sole goal is size. Relatively speaking, the subcutaneous plane is the "safe" plane.
With regards to your specific question, the procedure is clearly high risk. The most significant of the risks is pulmonary fat embioli (as described above) resulting in pulmonary compromise or death.
In summary, the procedure can be safely performed, however, they should be performed by a board certified (ABPS) plastic surgeon with experience. These risks should be discussed as well as the strategies being employed to avoid them.