As a transgender mtf, how would a large hair transplant done before HRT be affected by estrogen and testosterone blockers?

Transgender hair transplant surgery should be of no issue while taking HRT or no HRT.  Testosterone blockers may help but your genetic predisposition of male pattern hair loss will eventually manifest regardless of HRT.

It is possible to undergo hair transplantation surgery before or after starting HRT. Hormones may improve your hair loss prognosis, but they will not affect your hair transplant results.

The answer in part comes from your current age and family history of male pattern balding. While the medications may help slow balding it's not possible to predict with certainly their effect on slowing balding. The only way to know is to try them. In general for MTF transgender patients I advise starting medications first and monitoring hair loss. Altering the hairline is the first priority but I only do this once I'm confident further recession behind it is not going to be an immediate concern should balding happen rapidly over the next 1-3 years. One must always keep in mind and plan for the possibility and necessity of further hair restoration procedures. Going slow with conservative sessions usually yields best long term results 

This can be done before or after the transgender surgery. The hormones will not impact the hair transplant results, so you can go forward on any plan that makes sense for you

Since you are taking testosterone blocker it may actually help. if you are genetically susceptible to male  patten hair loss testosterone in the blood can accelerate the hair loss, but a blocker may help re growth.