Cool the cakes in the pans before frosting. If making the cakes in advance, wrap the cooled cakes individually in plastic wrap and keep at room temperature overnight, or freeze for up to 3-6 months.
In the bowl of your stand mixer fitted with the whisk attachment, beat the butter for 1 minute until smooth. Beat in the blueberry powder or blueberry jam.
In a separate bowl, combine the powdered sugar, meringue powder and salt. With the mixer on low, add the dry ingredients by spoonfuls, mixing just until combined; the frosty will be clumpy.
Add the vanilla, and whip the buttercream on medium speed for 4-5 minutes, adding milk as needed for desired consistency, and scraping the bowl occasionally, until very light and fluffy. If you used jam, you will most likely not need to add any milk.
Place one of the cakes on a cake board or cake pedestal. Frost the top of the cake with a thin layer of buttercream. Use an icing spatula to create a "dam" of buttercream around the edge that's higher than the middle.
Spread a layer of jam over the buttercream, stopping at the edge next to the dam. Use about 1/4-1/3 cup of jam.
Stack the next cake layer on top, and repeat with another layer of buttercream and jam. Stack the last cake layer on top.
Frost the cake all over with a thin crumb coat of buttercream, then refrigerate for 30 minutes to chill the crumb coat. Keep the bowl of buttercream covered, so it doesn't crust over.
Frost the cake all over with a final coat of buttercream. If you like, dab a little jam onto the buttercream and swirl it in.
For a complete list of tips for baking cakes at high altitude, or adjusting for your own altitude, please see my Baking FAQs.