This has nothing to do with the shell you're using, but what parameters your mailx command supports. Check the man page for your system on whether or not it supports the '-b' option.
I checked the man pages for mailx and I do not find -b in the flags section. However I found the following. Please let me know if this can be used and how.
Mail Editor Subcommands for the mail, Mail Command
By default, the Mail program treats lines beginning with the ~ (tilde) character as subcommands. The following list
describes the subcommands used while in the mail editor. The editor recognizes subcommands only if you enter them at
the beginning of a new line.
~b AddressList
Adds names in AddressList to the list of addresses to receive blind copies of the message. The ~b subcommand
can only be used to add to, not change or delete, the contents of the Bcc: List.