If using fresh blueberries or rhubarb, lightly freeze while you get on with the recipe - the fruit ideally needs to be cold and dry but not the end of the world if not.
Cut up butter into 1/2"-ish cubes. Place in freezer.
If baking same-day, preheat oven to 425F. Place rack in middle or upper third of oven. If your oven tends to run hot on the bottom, bake in upper third - and ditto if you oven runs hot on top - bake in middle.
Tip: scones are best baked very cold or frozen. If baking same-day, I recommend you place the raw scones on a 1/4 sheet tray or plate that fits in your fridge or freezer - think ahead if you know your baking tray won't fit in your fridge or freezer. I also recommend baking on TWO sheet trays, to prevent scone bottoms from scorching.
If freezing and baking later, prepare a plate or 1/4" sheet tray with parchment. You will set the scones on here, freeze all the way, then place into a bag for storage.
Pour 1 cup of heavy cream into measuring cup. Get a fork. Set aside within reach on your work station.
In a large bowl, whisk the flours, sugar, baking powder and salt. Add in the butter, and cut in using a fork, or ideally your fingers. Press the cubes flat, then massage the butter into pea-size bits. It's OK if some larger butter pieces remain.
Dump in the cold or frozen blueberries/rhubarb. Toss to coat in flour. Drizzle in 3/4 cup of heavy cream, and using the fork, mix into a shaggy mess. If the dough is not coming together after a few mixes, add in the remaning 1/4 cup cream. If needed, add in 1-2 TB more after this addition. You want a firm dough, not overly-wet. You should be able to mostly form the dough into a loose ball in the bowl.
Lightly dust counter with flour, and pour out the dough. Gently knead, to only get the dough into a 1 1/2" to 1 3/4" thick disc. Cut the disc into 6 round-bottomed triangles.
Transfer the scones onto the baking tray or plate that fits into your freezer. If you are freezing for use later, allow to freeze until solid, then place in a zip-lock bag.
If baking same day, allow scones to chill 15-20 minutes. Then, place on the lined baking tray prepared earlier, optionally using two baking trays stacked on each other to prevent bottom scorch.
Brush tops liberally with cold heavy cream, and top each with a few healthy pinches of the demerara or turbinado sugar.
Bake for 20-25 minutes, until the scones are puffed and golden brown. Allow to cool slightly before removing from sheet tray. Enjoy warm with a large cup of coffee or tea.