More powerful magic is easier to spot with the gathered mana normally appearing as a disturbance or glowing aura in the air.
So, mundanes do in fact have the ability to sense magic, so the blind person thing has no real bearing on it. Anyways, it's more like having a blind (or sight impaired) person describe what it was that happened to them. They can't directly tell that it was a car, or what color it was, because they couldn't see it directly. However, they could hear the wheels screeching and were near a street, and can infer the car. Similarly, mundanes can see visual distortions and glows when powerful spells are in play, and though they can't really describe what the spell is (having no way to directly assense it), they
can figure out that it's a spell.
Similarly, even spells that don't have specific visual effects
do have visual effects, via the above quoted rules!
Specifically in the rules, it says that a part of the 'detect magic is happening' roll is based around seeing the caster in the process of casting, noticing the eye contact and gesturing when appropriate (though casting in no way requires gestures or muttering, silly writers!). However, there are rules for detecting ritual magic targeted at you, and it's likely you aren't in line of sight of the caster or anyone obviously casting magic! That, taken together with the section I quoted in this post, indicates that the mana itself (at significant concentrations) is visible to some degree.
It's just the same as heat. (Remember that while magic doesn't follow the normal rules of science, it follows its own pseudoscience that mirrors true disciplines of normal science.) We lack thermosensing eyes, to directly see heat and heat transfer. However, if there is enough of a temperature difference in the area around us, we can see a "heat shimmer"! In this way, though mundanes cannot directly sense the mana involved with a spell, when there is enough of it concentrated into a spell, they are able to see 'mana shimmers'.
The second question is if this mana shimmer is only seen at the moment of casting, (when released) or if it's any time mana is accumulated. Again, you have only to turn to the ritual spell noticing rules to infer the latter. You may sense a ritual spell in the works before it is unleashed on you, you (And those around you) sense or see the very thickness of mana in the area as it slowly builds up.
Take these two together:
1) You can see accumulated mana, even if you cannot directly detect mana, by observing its secondary effects. Like a 'mana shimmer'.
2) You are seeing (visual manifestations of) the mass of mana itself, not the release of the mana.
And apply them to this situation:
A) A high concentration of mana is being maintained in a location.
And you get:
A.1) You can see accumulated mana. Therefore you can see this mana.
A.2) You see mana even if it's not in the act of being released. This mana is being sustained, but you can still see it.
And anyhow, if it's something like a deflection spell, it's actually constantly acting on the air around the caster. So point A.2) is kind of moot, you're still seeing magic as it's actually acting.
The rules on this aren't exactly clear without some thinking through it all, so it is really up to the GM on the matter (as it always is anyways), but there is call in the books for a ruling from a GM to say that a (significantly large) number of sustained spells would become apparent to mundanes on proper scrutiny, without having to resort to actual houseruling.
EDIT: A few spot edits for clarity.