I would say that you actually should have the The. This could sound crazy, but go with me here.
I don't think blocking is really the subject of the being verb. From a strictly grammatical standpoint, it is nominative and it is the subject. However, you aren't really talking about blocking. What comes after it, I would argue, is as important as the blocking part itself. It's really an inextricable part of the subject. If this were a more inflected language (i.e., one in which word order has more to do with emphasis than grammar), the whole prepositional phrase would be in the attributive position, so that in a very real sense it would be a part of the subject. In Greek, for instance, you would wrap that in an article-gerund sandwich:
article [attributes] referent
and that entire thing would be the subject.
Usually, English doesn't feel the need to express that explicitly. We normally just get it. However, this situation is complicated by the fact that the main subject is a gerund. So, in order to confuse us less, we make the main subject something that we're used to (an article), and let it do two things for us: first, force us to think about the word "blocking" as a noun, and second, wrap the gerund and the prepositional phrase into one big happy subject.