If you're having a hard time with blush application, the one brush you absolutely, positively, no doubt need is the MAC #187. Don't try to 'cheap out' on other, less expensive 'skunk' style brushes, 'cause (*I* feel) they don't measure up. Yes, the 187 IS expensive..but I've had mine for about two years now, and it's good as ever. I LOVE this brush. Aaand, blush isn't the only product this brush is good for. It has multi-uses, some of which include powders (pressed and loose), Skinfinshes (natural and mineralized), Bobbi Brown Shimmer Bricks and similiar products, any type of luminizing product (liquids, creams, and powders), and of course, foundation..I'm sure by now you've heard of the wonders this brush can do with foundation; making it look "air brushed", and flawless. So, really, this brush is every bit worth the money. I'm going to call it a must-have. If you tend to be heavy handed with the product, it won't matter with this brush, because the bristles are designed so that it only picks up what you need..and then it stipples the product on your face. It makes it so easy for beginners that aren't sure how much to use, or how to use it.
Another great brush is the MAC #182 Kabuki. I love this one as well, and I use it for powders and blushes. It picks up a good amount of product, and if you get too much, you can gently tap off the excess. Then, using small, circular motions, 'tap' the brush where you want it. It delivers such a great look. I particularly like it for blush, because it gives that 'rosy' cheek look.
The other brush that's good is the MAC #129. This is a 'standard' makeup artist-line based blush brush..You can get the long or shorter handled version (although I prefer the longer handle). This one is a little trickier to use if you are a novice...but defenitely belongs in a brush kit. This brush is good for powder, also, but the head is much smaller than a standard powder brush, and made mostly for blush and Beauty Powder.
My advice would be to first pick up the #187...THEN, if you want to you can get the #182, and #129..in that order. You'll do best, I think, with the 187.
Hope that helped you!! Good luck!! Happy blushing!