Is there a better way to get blueprints?

In 4 days of mapping I have had 1 drop.

What am I doing wrong?

They drop anywhere off of anything. Story/Maps/Delve/Contracts.

The only thing you can do it get more quantity.
i.e Alc your maps.

You can try buying some. Grand Heist are not that rewarding except the last chest.

I get a blueprint every 10 contracts or so, in practice it is slightly more because you also drops some in heists.

Quantity helps a long way for this kind of things, as a tip I suggest you putting item quantity for heist on your rogue talisman and on the talismans of your crew to get a bit more of it (and everything)