The Infiltration trapping has two separate sets of text; you've correctly identified both, but let's examine both separately.
Given an opportunity to case an intended target...
you can invoke known aspects on the scene.
So you use Casing to create / discover an aspect, then Infiltration to use it. The first time you do this would be free, under the usual rules for a free tag of a newly revealed aspect. (Remember, Casing is a type of assessment, so it's immune to the usual time limit for free tags - you can survey the scene well in advance, then tag it when you need to.)
So yes, this is just like tagging aspects in every other situation. The first two sentences are really reminding you of a benefit you already got with Casing.
You can use your Burglary skill to complement any skills you use on targets you've had a chance to study and prepare for.
Note that "complement" means that the skill can only increase, not reduce, your skill (see YS p214). So you'll get a +1 bonus to your other skill if Burglary is higher.
Now we can go back to your example:
Let's assume there is a camera with a Blind Zone aspect.
If you have learned about this aspect by Casing, then Burglary complements Stealth. So if your Burglary is better than your Stealth, you get to improve your Stealth roll by +1.
If you have learned about this aspect by any other means that didn't give you any preparation time (such as "someone told you about it"), you can't use Burglary to improve your Stealth roll, because you haven't had a chance to "study or prepare for" the target. If there has been a chance to prepare - someone told you days in advance, and you've studied it yourself - then you'd probably still get the +1 bonus.
(If your Burglary is worse than your Stealth, then no bonus is possible either way.)
- Besides this possible +1 from Burglary, you may also tag Blind Zone as usual. If you discovered it yourself with Casing, that means the first tag will be free.
So the bonus ranges ranges from nothing (if someone just told you about the camera) to +3 (if you Case it yourself, and your Burglary is higher than you Stealth, and you use your free tag.)