I think she didn't care. Like she checked to see if he would be a direct threat to their mission and then that was enough - he wouldn't be. At that time, Leiliana was pretty much "whatever the cost to get it done". Both Solas and Blackwall she was like 'they seem harmless enough for now and we need them'. Corypheus was way more pressing. Frankly, she would probably defend her decision even AFTER finding out what he was. She would likely not apologize for it, nor should she. I would have done the same thing. Solas was the only person to know what was up with the Fade. Blackwall is a good fighter.
Why waste good resources? ****, I would have recruited the blighted magister if I could.
Well, I don't think it was her call. She gave the Inquisitor the sword which showed that she meant to follow the Inquisitor. It should of been your Inquisitor's informed choice, and just suppose Leliana was wrong about the character of Blackwall and Solas. They were both optionally very close to the Inquisitor and could of caused a lot of damage had that been their intention.
The biggest thing though is it should of been your choice. These were people you traveled and fought with, and optionally bedded. Nomatter how honorable they seemed, your past and *definitely* your honesty should and do effect how much faith people put in you. Especially, especially the former. Leliana was the spymaster, not the Inquisitor. It wasn't her decision, no matter how she perceived the characters it wasn't her call. These were the Inquisitors companions! They made decisions depending onwhqt they thought was going on with their companion. This is especially apparent with the Blackwall romance. Not every Blackwall romancer was able to forgive him. Real life me wouldn't, my Lavellan wouldn't (she is definitely a hypocrite though because she's almost certainly going to forgive Solas X). Those Inquisitors took an emotional blow that was, if not completely unnecessary, could of at least been softened by being able to confront Blackeall before he f***ed and ducked.
She might have assumed the IQ wouldn't care given all the other shady types they're perfectly fine with. Or she might not think she has to tell them everything. She doesn't even have to be very happy with the IQ being in charge. My point is, one could come up with many reasons of why she didn't tell other than she's too soft or incompetent.
She shouldn't have assumed and she chose to follow the Inquisitor. Already pointed it out above,but your Inquisitor was the one traveling (bedding) these people, not Leliana. Your Inquisitor should have been told. Leliana really messed up, as a spymaster. And the decision to keep this information from the Inquisitor, among other things, makes her an inept spymaster.

I love Leliana as a character, and she did a lot for the Inquisition, but she made a lot of glaring mistakes. Better pack her off to be divine and find someone a bit more detached to play your shadow.