In Dutch we use the translated equivalent of not hindered with any knowledge to indicate somebody without any knowledge on the subject. It is not necessarily negative. It can mean that somebody is complete open and new to the subject, which may lead to new ideas, insights etc. But in a different context it may also carry a sarcastic tone indicating somebody with the biggest mouth and slightest knowledge. Is this equivalent comparable and usable or are there better constructs?
Normally a person not hindered with knowledge is called a blank slate (which is the English translation of Tabula Rasa), but it usually doesn't have the sarcastic overtones.
I personally like the construct you used.
Not hindered with any knowledge
It could be either a biting, witty reply or a serious description without any overtones.
Another way of stating the same thing is as follows:
Unfettered by knowledge.
The phrase blank slate is a good match to your first meaning. It does not necessarily carry any negative conotations.
For the second meaning, I would probably use the word clueless. It doesn't imply a big mouth, but it clearly states that the person you are referring to is so lacking in knowledge that they may not even realize how little they know.
I could also refer to myself as clueless on a given topic. That does not carry the same negative conotations as when I use it referring to someone else. It would in fact be a good match for your first meaning, but only when the speaker is referring to themself.
In English, I would expect that phrase to be used mostly, or even entirely, in the sarcastic sense: "My manager has a unique ability to make a decision unhindered by knowledge" -> he routinely tells me to do things that I know make no sense.
For the sense of someone simply entering into a completely new realm of learning or experience, adding a word or two to make it clear that you're talking about their knowledge up to now might be helpful: "He started learning about the new software unhindered by any prior knowledge of its quirks and foibles" or "As a new user unhindered by prior knowledge, John has a unique ability to ask questions that make us rethink our assumptions."
If you are looking for a single construction that can convey both ideas, you could try the adjective ignorant. It is used in an impersonal sense to denote a lack of knowledge about a given subject. But it can also be used acerbically to imply a lack of knowledge about everything.
I think the closest equivalent we have in English is the concept of somebody "knowing just enough to be dangerous". This is used for people who perhaps aren't experts on a subject, but know enough to think they know what they are doing. People who think they know what they are doing tend to lose the caution a neophyte would posess.
More accurately your "not hindered with any knowldege" person we might say "doesn't* know enough to be dangerous".