I had the same question earlier this summer, when we were barbecueing some fish fillets and I wanted to introduce some blackened fish type stuff. I searched around the net and came up with a mixture, something like this
(I live outside the us as well and something like "cajun spice mix" is unheard of here...)
1 garlic powder
1 onion powder
1 black pepper
1 white pepper
1 crushed oregano
cayenne pepper and salt, as much or as little as you like... taste to determine the amount.
(numbers indicated... proportions. can be converted using weight, tsp, tbsp etc.)
It worked out well!!