Which is considered safer - tummy tuck or BBL? I'm concerned about blood clots.

Hello,Although the tummy tuck has a higher rate of blood clots, only a fraction become life threatening emboli that lead to death (clots that travel upstream to the heart and lungs are known as emboli).  Fat grafting of the buttocks has a lower rate of blood clots/blood clot emboli, but has a higher risk of fat emboli leading to death. Although BBL is all the rage, it is the most dangerous cosmetic operation, so much so that ASPS has created a task force to investigate why and how to make it safer.  This is something people have not been hearing about enough. 

Regarding blood clots, a patient would be more at risk with a tummy tuck than a Brazilian butt lift (BBL). This is because of the increased tension in the abdomen from repairing the separation of the rectus muscles (the six-pack muscles). However, for decades of performing tummy tucks, I have had only one patient with a pulmonary embolus, and she subsequently did fine.
Early mobilization of patients is key to reducing the risk of blood clots with TT's. Also other measures are used such as sequential pressure hoses, which keeps the blood flowing nicely in the legs and pelvis. Mobilizing the patient early is easier with the No-drain TT, which I highly recommend.
The emboli problem with a BBL is fat emboli. There have been a fair number of these reported in the recent past and several plastic surgery panels discussing the problem. It appears to have occurred more frequently with assembly-line be BBL's, using cannulas that are too small, injecting fat too deeply into the muscles with the cannulas, entering from the wrong direction, and not being careful. 
The risk of both fat and blood clot emboli can be minimized when great care is taken with any procedure and recommended protocol is followed. 
Thank you for your very interesting question. I hope my answer has helped you somewhat.

As with any surgery, there are risks and benefits.  Both BBL and tummy tucks have been associated with emboli that can result in death.  Fortunately, this is a rare event but it IS a risk.  There are other risks associated with these procedures as well.  I would suggest that you schedule one or two appointments with a board-certified plastic surgeon who can examine you in person and review these risks and your other options with you.