Are people in a psychosis blind to everything around them? does it take them a while to understand things (even the simplest things?)?

No absolute rules: Psychotic people vary widely. Some may be very unaware of their environment, some partly or intermittently and some may be quite aware. Same w/ comprehension. You are right to understand both as very possible effects of psychosis, but not absolutely necessary. If these are happening, intense treatment and a safe setting are essential whatever the exact diagnosis. Take care - be well.

Not total blindness: Although patients claim total blindness they can usually walk without hitting objests and can take of their personal needs.