Drop single request based on header value

Yours should work, but here's a minor modification:

when HTTP_REQUEST {
    if { ( [HTTP::header exists True-Client-IP] ) and not ( [HTTP::header True-Client-IP] equals "127.0.0.1" ) } {
        set trueIP [HTTP::header True-Client-IP]
    } else {
        set trueIP [IP::client_addr]
    }
    if { [class match $trueIP equals block_list] } {
        discard
    }
}

You're allowing the client, or perhaps an upstream proxy, to set a header (True-Client-IP). If that header exists, use it in the class match, otherwise use the client's IPas seen by the F5. If a match is found, discard the request. Is that your intention?