Hello Stephan! Maglula did not design this strip magazine loader for 300 Blackout, and they don't explicitly recommend it for that cartridge. That said, they do report that some shooters have told them this 223/5.56 loader also functions well with 300 Blackout.
Like the manufacturer, I would not promise that this loader will work for 300 Blackout – but I too acknowledge that it does seem compatible with the round.