Well, there's the MAC 116 which is your standard everyday blush brush.
And then there's the 168 which is a contouring brush great for creating those "slimming" shadows on your face.
However, if you want more bang for your buck, I suggest you look into the following multi-purpose brushes from MAC:
-The 109 can be used as both a blush brush and a brush for liquid foundation application. Great for blending of the face in general. Small and can get into those crevices when applying liquid.
-The 187 and the 188 (smaller version of the 187) stippling brush is awesome and is a multi-purpose brush. It can be used for blush, MSF's, liquid or powder foundation, etc. Gives a flawless finish.
With any of these brushes, you can't really lose because even if they don't work out for your blush application, you can use it for applying something else.