Blueberry scone recipe calls for 3/4 cup heavy cream, can I replace the heavy cream with buttermilk?
Sure! It will be delicious, just a little less rich. I use milk in my scones and have used yogurt, whisked, in a pinch.
Buttermilk scones are awesome, you should! I've also used whole milk in the past.
If there is any baking powder or baking soda in the recipe...I would expect to add somewhat less baking powder than the original recipe prescribes, because the buttermilk contain sufficient acidity, and also add a small amount more baking soda as the buttermilk will react with more of it. There are some baking guides online with specify rules of thumb.
I make this substitution fairly routinely in scone recipes, and I don't think we miss the extra fat at all.
You can also, if necessary, use full fat yogurt.