Blockers tend to inhibit both sex hormones so masculinization might not happen or would proceed at a slower rate. In addition, the human body converts Testosterone to estrogen and that's why the dosage give to FTMs has be very carefully controlled. The blockers should have some effect but being in puberty for that long, it's hard to make the call without a close up examination. X rays can tell if bone development will continue and for how long and the blocker will prevent additional feminization. To give you more information than that would be guess work.
