The frosting is a Swiss meringue buttercream that is light and not too sweet. You can sub blackberry jam if you don't have fresh blackberries, but unless it's a seedless jam, I wouldn't go there. The seeds would definitely detract from the smooth fluffiness of the buttercream. Word to the wise -- this is not a frosting I would make while wearing my favorite blouse. Blackberries will stain everything, so be careful to keep the puree off your counter top and don't be surprised if it turns your rubber and wood utensils a nice shade of purple.
Meringue is an interesting animal and a little fussy to make if only because you have to make sure everything you use is completely grease-free. Which means -- metal or glass bowl, no rubber spatulas that have been used with butter or oil in the past, etc. I only have plastic whisks so I had to hold my metal whisk attachment for the mixer by the base and use that to whisk over the stove, which was a pain in the butt and very messy.
But it's all worth it in the end if hubby's eyes roll back in his head and he mumbles with his mouth full of cupcake, "Oh my God, that is so good!"
P.S. If you ever wondered what the difference is between Swiss, Italian and French meringue buttercream, this post breaks it down for you.