In-App purchases are only available for iOS 9.3.5 or higher/on Android OS 4.4. or higher and they can't be transferred between any of the different platforms, i.e. iOS/macOS/Android.
To purchase the Blues Styles, click on Settings/Extra Styles in iReal Pro:
Once you're in the Store, tap on Demo a to listen to some samples.
Tap on Buy.
Note: If you have already purchased the In-app, you will not be charged again to download it to your other iOS or Android devices after the initial purchase on that platform. Click on Restore Purchases. Make sure you are logged in with the Apple ID or Google account you used for the original purchase.
The PURCHASED button will indicate that the in-app was purchased and installed.
The Blues Styles will appear in your playback style list in Player controls under Blues.
Go to a song, tap the playback style.
See examples below:
In case you still don't see them, check if you're looking at the right place: