How can I make it so when a player is in a certian area only that players screen gets blurred or their line of sight goes down?

Yes, very long title.

As the title states, How can I make it so when a player is in a certian area only that players screen gets blurred or their line of sight goes down?

Use this for the areas: Introducing OverlapParams - New Spatial Query API

And use this when a player entered the area: BlurEffect | Roblox Creator Documentation

Adding on to what @BobbieTrooper said for the FOV use Camera | Roblox Creator Documentation for line of sight