Will a BBL mess up my large tattoo that goes from under my armpit to my back?

Hello! It is difficult to say without viewing photos. It shouldn't affect your tattoo much but if you do not have great skin laxity you may be left with some wrinkling\loose skin. Be sure to consult with a board certified plastic surgeon. Many clinics are offering virtual consultations at this time. Best of luck. 

Hello and great question. I appreciate the time you are spending researching this procedure. My name is Dr. Matthew J. Nykiel. I am a Board Certified Plastic Surgeon, a Fellowship Trained Cosmetic Surgeon, and a Butt Augmentation Specialist.
There are no major incisions in liposuction and fat transfer, your tattoo should remain unchanged.
I would definitely recommend doing an in person consultation with a board certified plastic surgeon who specializes in this procedure and who can suggest what is needed. I hope that helps! Best of luck!

Hard to predict without photos; if your skin lacks elasticity, there may be some stretching. The scars are small, but some surgeons will make many more incisions and in less discrete locations. Depending upon how much fat is transferred to the buttocks, the tattoo size may be altered.
During your consultation, discuss your concerns about preserving your tattoos with your plastic surgeon. 

Dear azizamichaelson,
since there are no major incisions in liposuction and BBL, your tattoo should remain unchanged. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Hello, it is difficult to say an accurate comment without seeing photos. It should not affect your tattoo much, If your skin lacks elasticity, there may be some stretching. I recommend consulting a board certified surgeon.

A BBL should not affect a tattoo on your back. A Skype or FaceTime consultation with a BBL specialist can help you get an accurate recommendation and pricing. 
Respectfully, 
Arian Mowlavi MD, FACS
Realself 100 Hall of Fame Inductee

Hello,

Thank you for sharing the information, it is a pleasure for me to answer your question.

It is difficult to evaluate the options without seeing photos,
It is advisable to consult with a certified plastic surgeon, for a good evaluation, to provide you with all the information you need to know about the appropriate recommendations, so that you can achieve the desired goals in your body.

I hope this answer will help you

My best wishes for you...