T-shirt because it looks like the letter T. They became popular during and after World War Two when American military men wore them openly, not under another shirt. They were usually worn under a shirt, which is more formal, a dress shirt, a sport shirt, shirts for business, military uniforms or more official occasions. That is what a shirt is. But after the war Americans started wearing them with no other shirt over them. Blouse is usually for a woman. Amul from United Kingdom gave some good photos of blouses. There are many names for the clothing Americans wear, especially women. Blouse is just one word. There are many more. If you are interested in fashion and all the different names of the clothing, you should so some more research. Good luck to you!

Shirts, t-shirts are unisex but blouse is a garment worn by women. Moreover in terms of fitting blouse is body fit, t-shirt is close-fitting and shirt is loose as compared to both. Hope I've answered your question.

T-shirt https://encrypted.google.com/search?q=t-shirt&sa=G&tbm=isch&tbo=u&source=univ&ei=1zACU8GYFoWihgfq7IHAAg&ved=0CEkQsAQ&biw=1920&bih=929&dpr=1 Shirt: http://www.topman.com/en/tmuk/category/clothing-140502/mens-shirts-140515?geoip=noredirect Blouse: https://encrypted.google.com/search?q=t-shirt&sa=G&tbm=isch&tbo=u&source=univ&ei=1zACU8GYFoWihgfq7IHAAg&ved=0CEkQsAQ&biw=1920&bih=929&dpr=1#q=blouse&tbm=isch

T-shirt = no sleeves shirt = for men blouse = for women