A blouson top is mostly a loose fitted top that may or may not cover the hips. It is not so tailored as a shirt, hence easily adaptable for various body types. Blouson can be various varieites and they were actually a replacement of shirts for women. Blouson are not very fitted by style, hence there is an ease to the wear, unless you opt for the stylised version like a peplum blouson.
A shirt is a tailored top, with button opening at the front from collor to hem. Since it is tailored than a blouson, it has restriction to wear for women with heavy bust / midriff variation. Shirts were traditionally part of men’s wardrobe, they started seeping in women’s wardobe in 1950’s .