When you are family sharing icloud and you block a number on one device, does it block the number on all devices?

Blocking of numbers is for an individual owner of the device , if the family organiser has created a family sharing see this article Family Sharing - Apple Support

They can locate the missing devices , and blocking of family member is done individually see this article Block phone numbers and contacts on your iPhone, iPad, or iPod touch - Apple Support

So , when the family organiser blocks his phone it will be blocked for himself and it will not effect other members , same is applied for other members ( they can also block if they desire ) .

And the simple answer for your question is " no " .

If you are sharing an Apple ID, blocking on one device will block on all of them. But, as LarryHN says, it shouldn't work that way with Family Sharing and is easy enough to test.

probably not - give it a try and see

LN