The flowers are common. A lot of the veg sold with a name that is "[adjective] choi" (or tsoi, or cai, depending on whether you're working with Englishized Mandarin, Cantonese, or real Mandarin) -- "choi" means "vegetable" and usually indicates something green and leafy, the modifiers can vary -- are sold when they're flowering as well as when they're not yet flowering. They're fine either way.

The flowers are usually white or yellow, more often yellow, and they're always edible. Some people don't care for the texture of the flowers and remove them. But you'll see them on gai lan, ta tsoi, weng cai, toongsin tsai, etc etc etc.