Once it blooms it turns bitter. Still edible, but a different flavor

Yes, you can eat the flowers. We combine them with regular broccoli in a saute. They make a pretty addition. The very end of the stems are also edible but anything more than an inch or two from the end can be woody.

The broccoli and broccolini in my garden have grown so big it's hard to keep up. This means I'll sometimes eat bits that have begun flowering. I haven't noticed much difference in the taste.