The slow blink is a cat's way to tell other cats I do not want to fight you.
So it is a way for cats to avoid trouble before it starts. If two cats meet and only one of them blinks the one blinking will retreat as this shows the cat not slow blinking will fight or protect its territory.
So the slow blink is to tell the other cat I am not hostile.
This is the same if you slow blink to a cat, you tell the cat I am not hostile. If the cat blinks back you both have a deal in not being unfriendly.
So this is not an invitation to come closer to the cat even if this is often possible.
In this situation let the cat come to you. If you try to approach the cat it will feel like prey and run away.
We people have the same type of non verbal communication we do often smile to unknown people we meet because we are unsure about the person. If the person smiles back it gives a signal of not being unfriendly.