Yes, I should have clarified, it does "block" estrogen. It probably should be clarified in full then if we want to be technical.
What Nolva does is block the estrogen receptors in the body and breast tissue. Although you may have "estrogen" present circulating in the bloodstream even while taking nolva, at any rate, it does not matter, it will be so little if at all, that very little or again, no estrogenic activity will be possible. If estrogen cannot bind to the receptors, it is not effective, aka it has ZERO effect.
Also we may as well explain for new guys an explanation to how arimidex works. Arimidex works by blocking endogeneous production of estrogen. So in theory it lowers the amount of estrogen that you create compared to nolva blocking and tying up receptors to make estrogen essentially non-active. Both are very good products, both "can" work very well.
I would argue (and this is my opinion) that Nolva in my experience is better for removing pre gyno and existing issues with the nipples, etc..and/or controlling pregyno symptoms, whereas arimidex has a better overall effect in a lower total estrogen level, however I have found it is not near as strong in removing pre-gyno symptoms as Nolva. There is also Letro as well that is popular.
Estrogen reduction can be accomplished in several ways – by blocking it, by destroying it, by occupying existing estrogen receptors