You could use a controlled release but don't use one where the nitrogen is largely nitrate. I've been able to grow them using just the ammonium sulfate. But don't use too much or your media can become too acidic.

I assumed as it was for Azaleas, Rhododendrons, etc it would be ok. Is that not always the case?

Yes, it should be fine but still check the label. You don't want to see nitrate as a source for N.

In terms of the amount that I should use, as it is for Azaleas etc do I just feed every 3 months, or do I need to feed less for Blueberries?

Containers error on using less. In ground you can just follow the directions. That is usually three applications of CRF starting a little before your last frost date. I do that and still use ammonium sulfate if growth isn't as vigorous and/or green enough.

Do a google search for growing blueberries in your country to get better recommendations. You may need to adjust based on your local climate and soil.