Best way to blanch it: Boil water, pour it on a slice or chunk of zucchini. Let it sit for 5 minutes. Take the zucchini out and put it in ice water. Drop it in the tank.
As for how much. This going to depend entirely on the fish. My 40 gallon with my mollies will eat an entire full-sized zucchini in minutes. I have otos that will nibble on a slice for about an hour and then ignore it. My 20 gallon with mollies won't touch zucchini but will eat the hell out of summer squash. (I do not understand my fish. If I take the fish from the 20 and put them in the 40 they'll like zucchini.) So, put a half a zucchini in a tank. Take it out after 4-6 hours if they don't like it. Also try summer squash or cucumber. None of my fish like cucumber particularly, but it's worth a shot.