Nope, That Is Not a Blouse: Simplicity 9817
This set of re-issued accessories from the 1950s has some expected items and some unusual ones. Collars and cuffs that could be switched out, and thus give some variety to one dress, were a standard feature from at least the late 19th Century onward. Here, you see two rounded versions and one designed to look like lapels on a blouse.
Hair bows too date way back although this is a bow on a headband which was a newer look in the 1950s. The bias-cut sash, which you see on the bottom row in red and in green stripes is also a traditional idea and here it is pulled through a buckle.
A newer idea was the dickey that looked like a blouse. These were often recommended during World War II when fabric was hard to get. A woman could use just a small bit of fabric, maybe a remnant from a bigger project, and could give a new look under a dress that could not be replaced. It was also possible, as it is suggested by these illustration to wear a dickey under a suit as you see in the lower right hand corner with a two-color version. Yes, it looks like a wrapped blouse or a halter top, but this item actually has back and no sides and ties at the waist. I am not sure I see the point unless you had a desperate lack of fabric. Wouldn't it be no more difficult to make a halter top, or a sleeveless blouse? And surely, wearing an actual top would be more comfortable and less awkward, and allow you to doff your jacket if you liked.
You can find this pattern at your local fabric store or online here:
Simplicity Sewing Pattern S9817 Misses' Neckwear, Headband, Dickey and Sash-Belt