In some situations, it doesn't hurt to do a fresh re-installation.
Close your League of Legends Client.
Open your Windows control panel, move over to Uninstall a program, select Blitz and click Uninstall.
Press Windows Key + R, enter %localappdata%/Programs, click OK and delete the Blitz directory.
Press Windows Key + R, enter %appdata%, click OK and delete the Blitz & Blitz-helpers directory.
Now head over to blitz.gg/download, download and install the latest version.
Open your finder, press Command + Shift + G, enter ~/Library/Application Support/, remove the Blitz directory.
Open your applications directory and delete the Blitz.app
Now head over to blitz.gg/download and install the latest version.
One-liner that you can enter into your Terminal on macOS to do step 1 and 2 a bit quicker.
osascript -e 'quit app "Blitz"' && sleep 1 && rm -rf ~/Library/Application\ Support/Blitz/ && rm -rf /Applications/Blitz.app/