Lemon Cupcakes with Raspberry Frosting

In a month or so, I'm catering the cupcakes for a wedding again, a neighbor. She wants pretty plain cupcakes - vanilla, chocolate, and lemon. I asked what frosting she wanted with the lemon and she wasn't sure, so I offered to make a raspberry frosting for her, to see how it worked. I did that today. as a trial to see how it tasted. Here's the recipe I used for the frosting:
(for the cupcakes, I literally just cheated and used a box, adding 2 TB of lemon juice to make it more lemony delicious)

1/2 cup fresh raspberries
1 cup butter, softened
8 oz cream cheese, softened
4+ cups powdered sugar (you know the drill, you be the judge when you start adding)

Puree the 1/2 cup raspberries in a blender. It's such a small amount that if you put it in a food processor (at least in my case) that after 1 spin of processing, it's on the side and the blades couldn't reach anymore. So basically, I dirtied the food processor for no reason. You should be left with 3-4 tablespoons raspberry puree. I left the seeds because I liked the look, and it seemed more trouble than it was worth to get them out. Beat butter and cream cheese together until smooth and creamy. Add powdered sugar a cup at a time while beating and cream. Add raspberry puree  and beat until creamy. I thought it was a bit runny, so even though I stopped adding powdered sugar (I didn't want it to be too sweet, a complaint the original blog was getting) it was a bit too...unstable. So I put it in the fridge for about a half hour before frosting, that helped. I think a cause to that was that I had my butter and cream cheese to soft. (while I was softening them, I realized I had to make a side for a BBQ I was going to tonight - so cupcakes were put on hold for a bit) So in the future, I would do that differently. And then I frosted these beauties, putting a raspberry on top.

Et voila. And then, the taste. The frosting is pretty good, despite the flaws. The cupcake is good, of course. But together? The aftertaste is...sour. It's not horrible but it just leaves you with a taste in your mouth of sourness. And that's all right. It's not the best, it's not the worst. I just know to improve from here!
(and as a side note, the bride-to-be has rejected this cupcake, wanting a different frosting. It's ok, that's about what I expected. We'll move on!)