If the phone gives you the option to format the card as internal storage then, as B. Diddy said, the question comes up. (If you don't get asked, the card is external storage and you fill it, the phone doesn't.)
But how it fills, whether it's random, flip-flop (one file goes to eMMC, then one to SD, then the next to eMMC) or some other method is up to Blu, not Android. (Some manufacturers don't even give you the choice, even if it's an 8GB storage phone (which is what internal storage SD was designed for), because SD cards don';t last as long when used that way.