Spiro is indeed a blocker. It is used most commonly in America, more out of tradition than being the most effective product on the market. It will block T, but it is also a diuretic, so you will have to drink a lot and pee a lot.
Dutasteride and Finasteride will block DHT, which is a more potent form of T, but will do nothing to stop standard T from affecting you (DHT is important in terminal hair differentiation - darkening, growing, etc. - so blocking it would have effects).
Whereas, flutamide, cyproterone acetate and bicalutamide will block both DHT and T (of these bicalutamide has the least side effects - I think by now everyone thinks I'm some sort of bicalutamide shill) however, I don't live in the US so I don't know which of these are approved by the FDA. Also, most of these blockers are not widely known as they are use more for Testosterone dependent prostate cancer.
It might be worth checking whether state laws mean you are able to seek out a gender therapist without them contacting your family for permission. Just because you see one doesn't mean you have to go on E, you can simply ask for blockers in the meanwhile.
TL;DR yes spiro is a blocker