There's a few factors to take into account.
1) Luck/RNG
2) Availability of other materials
3) Urgency of money
1) Luck/RNG:
It's obvious that how much money you'll earn from the blueprints relies heavily on which ones you get. You can get unlucky and get few flash bomb/radiation usages and gets lot of bad ones, making it a bit less profitable, or you could score quite a few good BPs and make a fortune. You probably would get more money by opening and making them than by just selling them, but remember your luck defines how much more money you'd make and how efficient it would be.
2) Availability of other materials:
If you have some choppers on trees, are able to gather elven fruit easily and farmed oysters in particular, making those blueprints will require less time and/or investment than if you'd struggle to get those materials or had to buy all of them. How many of those materials you have or how easily you can get them is worth taking into consideration as well.
3) Urgency of money:
Selling blueprints saves you a lot of trouble, time and patience. You simply find a seller or stall them overnight and gold appears in your inventory... aand that's about it. For manu you have to spend quite a bit of time opening, sorting and making the blueprints, and then selling the products, which (depending on what you made) can take more time than selling blueprints. But it does bring more money as we've concluded earlier.
My opinion/summary:
I've made this short analysis to help you decide by yourself, and you can use this for plenty of other methods in the game (sell sealed blueprints or open and cook/craft them, sell OSS or open them, etc.). But in short:
-Sell blueprints if you want small but quick money and not bother with luck or some time-consuming activities revolving manu;
-Open and make them if you want to get some more profit but also put in more effort and time into it.
I'd say just open them since in the end it's more profitable and honestly more fun to do manu than just kill oysters (or other mobs) for hours on end and sell their loot.
