Try to remember back when you were first getting the world in order in your mind. Remember how you learned to write, to draw, to read? Or how you got positional prepositions down?
Probably through watching someone and imitating them. You saw THEM put something in a box, you put it in a box. They showed you it, you did it. You saw them write the letter A and followed it up with a scribbly, freestyle impression of your own.
Now, what if you weren't able to imitate?
There's scientific evidence showing autistic kids have a problem in the 'mirror neuron' area of the brain. This area fires when watching someone do something, even if the something isn't actively imitated. But in autistic kids, it only fires IF they imitate it.
This is one reason autistic kids may not pick up on things that are second nature to others. How would you explain 'blue' to a blind person? They aren't able to implant that image in their brains. With autistic kids, things they may have absorbed through watching others doesn't set in. This also explains why so many autistics can't understand emotional expression - it doesn't connect with them, sliding through the brain.
Now, imagine putting together concepts like 'in', blue, the letter A and so many others simply by doing it yourself, not by getting it from your environment. For comparison, imagine putting together a computer from scratch with an instruction manual written in a language you don't understand and only a few pictures to guide you.
This also explains why the usual teaching methods have limited success with autistic kids. And why everything an autistic kid picks up should be impressive.