You can learn the difference between synthetic and natural bristles in the previous blog from this series - All About Foundation Brushes.
When it comes to blush brushes (try to say that 10x fast) it's all about the shape, honey. There is dome, angled, flat, stippling/duo fiber (which really isn't a shape, but I'm going to throw it in there - it's never fun to be left out), tapered, and there is probably another one or two that aren't coming to mind. I know for a fact that I own every version I just listed - seriously it's an addiction - each one will give me a different effect. My favorite shape is angled, and I actually have the Mary Kay angled blush brush that is my ride or die brush. An angled brush fits the contours of my cheeks and helps me precisely apply blush.
If you tend to be heavy handed with makeup application, a stippling/duo fiber brush will be your best friends. It doesn't pick up as much product as an angled or dome brush would.
Flat and dome brushes, like the MK Cheek Brush, are really amazing for multi-purpose use. You can use them on your cheeks, to contour, to highlight and even to apply your finishing powder if you just want one brush that does it all. These types of brushes are perfect for beginners too.
A tapered brush is really great for a smaller cheek area or to precisely add your highlighter. It could be used for contouring but it would a little more difficult to blend out product with this type of brush.
I lump blending brushes in with powder brushes just to give you a heads up. Every brush collection should have a designated powder brush. This is a brush that is big, fluffy and super soft, like the MK Powder Brush. This type of brush is used to apply your finishing powder to set your makeup. You really don't want to use your powder brush for anything else like contouring or blush because you don't want that product to build up on your brush and mess up your makeup.
If you do want something for blending your contour and blush, then a dome brush, similar to a dome cheek brush, is exactly what you've been looking for. Dome brushes tend to be more dense and make blending super easy. Like a finishing brush, you don't really want to use your blending brush for a lot of other things. If you get too much product build up in your blending brush, then it doesn't really blend out what is on your face, instead will just deposit more product and turn into a big mess. You don't want to be a total Monet right - #Clueless reference.
These types of brushes don't seem important at first, but they are usually your all-in-one brushes. Blush brushes can have many jobs depending on their shape. For your finishing brushes, these guys really shouldn't be used for other things than their designated job. Too much product build up in your finishing brushes will just make things messy. Remember to clean your brushes about once a week. For the next blog in this series, we're tackling eye and eyebrow brushes - oh, yes it's about to go down.