Edit. Rereading the question, I see that OP was using "blocks line of sight" in the colloquial sense of "unable to be seen through," not necessarily in the rules sense of "unable to be targeted by a spell." I'll leave my answer because I hope it addresses a possibly useful subquestion.
In the Players Handbook Chapter 11, Spellcasting, it says the following about spell targets:
A Clear Path to the Target
To target something, 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.
So it seems to be that the default rule for a wall is to block line of sight for purposes of choosing targets, unless the effect says otherwise — as other answers have pointed out it's the case with some other Wall of X spells.
Similarly, for areas of effect:
A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.
The rule for total cover given in Chapter 9 is:
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.