It's interesting that almost every point in the other replies is already explained in the article except yours.
I'm surprised the crows didn't spot Frodo as he approached Mordor. It's reasonable to assume they were covering more ground away from Mordor already though.
Dressed as orcs they probably blend in. If you're a crow patrolling the skies around Mordor, seeing a few extra (small) orc's wandering around on the ground probably isn't anything unusual or even your job to worry about. After all the entire place is ringed with natural defenses with orc's patrolling all the entries that do exist.
As a Crow your job is to watch the skies and make sure nothing untoward comes in that way.
It seems incredibly unlikely to me that Sauron is going to protect the borders of Mordor so well and then ignore the sky, especially when he has the perfect agents, the Crows, for the job.
I'm surprised the crows didn't spot Frodo as he approached Mordor. It's reasonable to assume they were covering more ground away from Mordor already though.