If the only sense involved is sight, then OK. But if the victim is hearing illusory screams and-or smelling illusory rot as well then being blinded might only serve to make the fear worse.
Per the 5e version of the spell, sight appears to be the only sense involved. Spell has a range of "self" (so the illusion is projected over your own body), and once any given target no longer has line of sight to you, they can start making saves to see if the effect expires (so it isn't instant, which is what I implied earlier, my apologies.)
To be fair, this is one way in which 5e differs from what I'm used to; in my case a Fear effect would have a set duration regardless of the relative subsequent positioning of the source and target, thus allowing the target to run around a corner or through a door and, still frightened, keep on running (or, if there's nowhere to run, cower in a corner) until the fear wears off on its own.
Well. The idea is that the fear
won't go away until the spell's normal duration
unless you can hide behind something.
Or casting any spell into darkness.
Assuming no darkvision, sure. Plenty of species in 5e have it (though not all!) and Warlocks can actually get super-ultra darkvision, which can see through both mundane and magical darkness (which no species provides).
Does it also say you need line of effect between the caster and the spell's point of origin, for targeted spells? Example: if I'm in my hous looking out the window and I see you skulking around in the garden can I drop a Hold Person on you? I seem to recall the RAW answer here would be "no I can't" but that's never made sense to me.
Well, here are the relevant rules. As noted, "line of effect" isn't a phrase the game uses. A few separate sections stitched together here for ease of comprehension. All emphasis in original.
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.
[...]
Areas of Effect
Spells such as burning hands and cone of cold cover an area, allowing them to affect multiple creatures at once.
A spell’s description specifies its area of effect, which typically has one of five different shapes: cone, cube, cylinder, line, or sphere. Every area of effect has a point of origin, a location from which the spell’s energy erupts. The rules for each shape specify how you position its point of origin. Typically, a point of origin is a point in space, but some spells have an area whose origin is a creature or an object.
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.
So if you are looking through a pane of transparent glass, then the answer is "no you can't". If you are looking through an open window, the answer is "yes you can". If, for example, there is a window already open, you could walk up to it, cast the spell, then use your one free "interact with an object" interaction to close the window and retreat. Other kinds of spells may or may not work that way, depending on the specifics of the spell in question.