It depends on the design of your database and what you are trying to accomplish.
Just a bit of a wild guess here as to what you are trying to do, but you could perform a find that omits all records where Blacklist contains the specified text.
#On a layout based on tbl_BlackList...
enter Find mode  ---> clear the pause check box
Set Field [Tbl_BlackList::blackList ; "*" & $URL & "*" ]
Omit Records ---> when in find mode, this step selects the "omit" option for the current find request
Set Error Capture [on] --> keeps an error dialog from interrupting the script if no records are found
Perform Find 
The resulting found set will not have any records where BlackList contains the data specified in $URL