Line a muffin tin with parchment paper liners. Preheat the oven to 350º F.
Add the oatmeal (from the muffin recipe, leave the oats for the topping whole) to a blender. Process into an oat flour. I leave the oat flour with some texture (whole oats), but grind into a finer flour if desired.
Into a medium bowl, combine the oat flour, almond flour, arrowroot, cinnamon, baking powder, baking soda, and sea salt. Stir to completely mix together.
In a larger separate bowl, add the eggs and whisk them. Then add the maple syrup, melted butter, Greek yogurt, and vanilla, and continue to whisk until smooth.
Add the dry ingredients into the bowl with the wet ingredients, and stir until just combined.
Fold half of the blackberries into the batter, reserving some to top the muffins.
Divide the muffin batter between the 6 muffin papers. Top each with the remaining blackberries, and 2 tablespoons of the oatmeal crumble topping.
Bake the muffins for 28-32 minutes, until the tops are lightly browned and the centers are cooked through.