[RPG] Does the Shield spell block the Jim’s Magic Missile spell, the same way it blocks the Magic Missile spell

I see nothing there that implies it is a barrier to vision, just an active and utter absence of illumination.

You are quite right - there is nothing in the spell that says it blocks vision, just that the area is in Darkness.

However, a strict reading of normal darkness means you can't see through that (PHB p.183):

Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.

And a Heavily Obscured area is (PHB p.183):

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A).

Which has been erratad as:

A heavily obscured area doesn’t blind you, but you are effectively blinded when you try to see something obscured by it.

So, darkness (magical or otherwise) creates a heavily obscured area. A heavily obscured area "blocks vision entirely".

Now, while it is clear what this means for "opaque fog, or dense foliage" is simple and straightforward - you can't see into this stuff and you can't see through it to stuff on the other side of it.

Applying this to darkness, however, seems to result in nonsense because, in the real world, darkness isn't a thing. In the real world darkness is the absence of light hitting your eyes from a certain direction. This can be because of an actual absence of light (underground) or because, even though the region is full of light none of it is coming your way (space). But this isn't the real world, is it?

So you have 3 options:

1. Darkness works just like it says in the book - you cannot see into it or through it. This would be really cool for a gothic horror campaign even though it would make navigating at night ridiculously hard.
2. Darkness (magical or not) works as it does in the real world - you can't see into it but you can see through it to illuminated areas beyond. I think this is what the rules intended even though they and the errata were poorly drafted.
3. Normal darkness works like 2. Magical darkness works like 1. There is no support for this in the Darkness spell description but this is how it worked in prior editions.

Its your world - make it fun.

It wouldn't.

A Magic Missile does 1d4+1 damage irrespective of the level of the caster. A higher level caster gets more missiles but intensify does not give the caster more levels except for increasing damage die which is not applicable to Magic Missile.