Do you need line of sight to cast spells on someone?

The rules on spellcasting contain the following section:

A Clear Path to the Target

To target something [with a spell], you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

This section is not really clear to me. Should this mean that only non-transparent objects are a problem in targeting, or that you have to have line of sight and line of effect as well?

Also, can you prevent a wizard from casting spells by blinding her?