Note: It's important that the bowl of your stand mixer is super clean with no grease residue or the egg whites may not whip properly.
In the bowl of your stand mixer, whip the egg whites until they’re frothy, about a minute. Add in the cream of tartar and sea salt then whip to soft peaks (a few minutes).
While they're whipping, add the freeze-dried blueberries to a food processor or blender and blend until powder form and no clumps remain. Once the egg whites are in stiff peaks, add the blueberry powder and whip again for a minute. Then you can turn off the mixer while you make the syrup.
In a medium/large pot, combine water, honey, and maple syrup. Set on the stove, turn the heat to medium and let it come to a boil. Keep it at a boil until it reaches 240ºF. Try to not disturb it, but if it starts to bubble too high, simply lower the heat just slightly. I highly recommend getting a cheap candy thermometer from Amazon. It's VERY important it reaches 240 to prevent the fluff from deflating later on.
Remove the syrup mixture from the heat. While the mixer is going on medium/high, very slowly pour into the egg whites. Once the syrup is all added, increase the speed and whip until the mix is thick and glossy (2-3 minutes).
Add in the vanilla and whip for another minute.Place in a large jar or container. This marshmallow fluff stores is best within the first 3-4 days but will last up to 1 week on the counter. You can always re-whip it if you feel as though it's gone flat, but i've never had to do so!