If you look at the "Note:" under $ibl(#channel/N), you will see the "$chan()" is a link. Click it and read the linked $chan() item.
topic, mode, key, limit, ial, logfile, stamp, ibl, status, inwho, wid, cid, hwnd
$chan(1).ibl returns $true if mIRC has already seen a /mode +b and has list of bans for the channel, or $inmode if a /mode +b is currently being listed. See the $ibl() identifier for more information.

If $chan(1).ibl != $true it stands to reason that the IBL would not be filled, and therefore that $ibl() wouldn't work. smile