can't read "Blackhole_Match"

That's very strange. The variable is created in the DNS_REQUEST event, so the error would indicate that this isn't happening for some random DNS_RESPONSE. There are at least two ways to mitigate this error:

1. Place a catch around the entire section of code.

   if { [catch {
       if { $Blackhole_Match } {
           ...
       }
   } err]} {
        some error occurred
   }
   

2. Perform an [info exists ] function before checking for the variable.

   if { ( [info exists Blackhole_Match] ) and ( $Blackhole_Match ) } {
       ...
   }
   

The question then becomes, what do you want to if an error occurs or the variable doesn't exists?