Bananas have many good qualities. It is low in cholesterol and sodium. It is a good source of fiber, vitamin C, potassium, manganese, and vitamin B6.1, 2
Te bad thing about bananas is that they can have up to 35.5g of sugar (fructose and glucose are the sugars most commonly found in fruits but no reference could be found to confirm which sugars are in fact contained in bananas). It is considered a high glycemic food and as such it should not be consumed by people interested in burning body fat.1
Never blend a banana. Bananas (as do apples, potatoes, and many other fruits and vegetables) oxidize rapidly. This has to do with an enzyme in the produce that react with oxygen in the air and iron in certain molecules inside the produce.3 Do you remember ever having cut an apple and left it resting in open air? It turned brown, right? That’s oxidation… Literally, rusting metal. The blender simply makes more cuts, exposing more surface area to the air and simultaneously liberating more enzymes.
It no longer has the benefits (the ions your body requires no longer exist) it once did and it could possibly be harmful to you (oxidation can be a free radical reaction).
Available at: http://www.nutritiondata.com/facts/fruits-and-fruit-juices/1846/2
 Dietary Fiber. WeHealNY.org.
Available at: http://www.wehealny.org/healthinfo/dietaryfiber/fibercontentchart.html
 Anne Marie Helmenstine, Ph.D. Why Do Cut Apples Pears Bananas and Potatoes Turn Brown? About.com
Available at: http://chemistry.about.com/od/chemistryfaqs/f/brownapplefaq.htm