Egypt's naval blockade was secondary. Israel struck preemptively mainly because it was clear that Egypt intended to invade, between the 100,000 troops they amassed near Israel's border, and Nasser's statements which left no doubt about his plans. The only debate is about how immanent those plans were.

It also seems misleading to talk about Israel's blockade of Gaza with no mention of all the rocket attacks that prompted it. A blockade might be an act of war, but it's sort of moot when it's preceded by acts of war from the other side.