You will have to ask the maker of the script you are using, or read it's help file (if it has one). Since there are no blacklists in default mIRC your blacklist option must be in a scipt you are using, and since we don't know what script that is, noone can help you.
To add someone to ignore though, which is built into mIRC, read this (from the mIRC help file):
The /ignore command
/ignore [-lrpcntikdwxu#] <on|off|nick/address> [type] [network]
Where p = private, c = channel, n = notice, t = ctcp, i = invite, k = control codes, d = dccs.
The -u# switch specifies a delay in seconds after which the ignore is automatically removed.
The -r switch indicates that the address is to be removed.
The -x switch indicates that this address should be excluded from ignores.
The -l switch displays the list of ignored addresses which match the specified switches.
The -w switch makes the ignore apply to any network.
If you do not specify a type then only the users nickname is used. If you specify a type then the users address is looked up via the server.
You can clear the ignore list by specifying -r with no address.
Note: If you have a /query window open with someone, private messages from them won't be ignored even if their address matches an ignore address.