fixed+Comments

local side = 'back' --Define rednet side
local IDblack = {6, 7}  --All blacklisted IDs
local black = {'Firewolf', 'DNS'} --All blacklisted messages

rednet.open(side) --Open rednet

id, msg = rednet.receive() --Receive the message

local detected = false  --Define a new variable called detected

print("ID = " .. id)
print("msg = " .. msg)

for i=1,#IDblack do --Do the following block of code Length of IDblack times. i stores the current run.
    print("IDBLACK" .. IDblack[i])
    if id == IDblack[i] then --If the current run found that IDblack contains an ID then do the following block of code
        for j=1,#black do   --Do the following block of code amount of entries in black times.
            print("black" .. black[j])
            if msg == black[j] then --If current run found that black contains the sent message then
                detected = true --Set detected to true
                print("Detected")
                break --And exit the loop
            end
        end
    end
    if detected == true then
        break
    end
end

if detected == true then --Do the alerting only if detected was set to true.
print('Blocked ID: '..id..' Detected... Denying Access')
rednet.send(id, 'You Are Blacklisted!')
end