PS: Just tried it,with an experiment comprised of my (many!) email addresses.
"test" message sent, using gmail, to a CC and a BCC.
The BCC could see the sender, as well as the CC. The CC could only see the sender.
"Reply to all" (using MacOS client) from the BCC went to both the sender, and the CC.
The main purpose of BCC really should be when you want to keep somebody "in the loop", allowing them to know what has been said about a larger group, without allowing others to know the BCC is in the loop. For example, I might (and have) send a complaint about a safety defect to an appliance manufacturer, BCC'ing a consumer body, such as BBC TTV 'Watchdog'. But in that example, if BBC Watchdog should decide to contribute then their cover is blown, that would be their decision.