The origination of "boyfriend" pieces of clothing came from the mindset of wearing whatever you feel like wearing. Clothing became analyzed for function and appeal on an artistic scale that didn't surround showing off the feminine aspects of a woman's body. Still, the "boyfriend" trend moved from actual men's closets to calculated designing, altering cuts for a woman's body. Wearing a men's blazer instead of a boyfriend jacket, for instance, is just more genuine.
Cut of a Men's Blazer
The requirements for a good fit in a men's blazer mainly focus on the close fit of the arm and chest areas. To start, the shoulder region needs to be tailored for full arm movement without an overdose of material; the widest points of the blazer's shoulders should not stick out more than a half-inch of the man's shoulders. The blazer's sleeves, meanwhile, are intended to show off roughly half an inch of a man's dress shirt cuff, so should fall at the wrist bone. As for a snug but breathable fit in the torso, you have both English style blazers which traditionally button starting at the upper chest and the American style, which starts around mid-torso. Either way, chances are that putting a men's blazer on a woman would most likely mean misfitting in the shoulder, arm length and breast region.
Cut of a Boyfriend Jacket
A boyfriend jacket or blazer was designed with a woman's frame in mind. You could even argue that the boyfriend blazer, while still playing the role of the over-sized outwear piece for women, takes the components of men's American, English and Italian style blazers that best flatter femininity. For one, Italian men's blazers are generally made with lightweight materials so that they fall with the wearer's shape. But even if made with heavier fabric, a boyfriend jacket would still be tailored to slope in the shoulders and tuck in around the waistline -- like the English men's blazer. Lastly, the buttoning would most likely start lower on the frame -- like the American men's blazer -- to allow room for the female bosom.
Theory of Androgyny
Androgynous characteristics in the metropolitan culture went from signifying you had no "real" role in society to being a positive sign of independence. In its earlier stages, androgynous dressing for women addressed that conforming to the norm -- when it came to more trivial things like how to dress yourself -- was not entirely necessary. When it finally evolved to be a chic thing to do, inspired designers made profit of enhancing what women wanted to wear into something that would better suit them, physically. But the philosophy behind wearing the opposite sex's clothes implies that you legitimately select an article of clothing meant for that opposite sex. For this reason, you can actually wear a men's blazer in place of a boyfriend's jacket.
Wearing a Men's Blazer
The consequential frumpiness of a men's blazer can be altered by the silhouettes of the rest of your ensemble. You may not be able to wear an actual men's blazer with trousers, as the pairing would make the whole outfit too over-sized, unlike with a boyfriend jacket. But you could wear it with either a very structured dress or a dress made of very fluid material, such as modal or chiffon. These two suggested silhouettes may contrast each other, in that one creates form-fittingness and the other creates great movement; but both would show off the womanly vibes that an actual men's blazer could not. The unshapely nature of a man's blazer on a woman, in fact, is the reason why many women either scrunch up the sleeves or pop overly-large lapels. All you have to do is style it according to what looks good on you.