I am trying to make something happen when you kill an enemy zombie?

So, in Roblox, if you write part.Touched:Connect(function(hit), everything that touches the part will be named "hit" including the basepart. a way to verify if the parent of "hit" is a player is to:

part.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") ~= nil then
        > YOUR CODE HERE.
    end
end)

What I did is that after something touched, if his parent has a child name "Humanoid" then it's a player's character cuz players's characters always have a child name "Humanoid".

Also, at line 5, you wrote game.Players.PlayerAdded:Connect(function(plr), and this will find the player, but this line of code is envlopped by the touched function and we don't know when the player will touch the part, but it's sure that the player touches it AFTER he has joined. So that line of code "game.Players.PlayerAdded:Connect(function(plr)" will NEVER run. You can find the player's name by getting the "hit" 's parent's name AFTER verifying it's a player. Then you use that name and get it in game.Players cuz the player's character's name is the same as the player's name in game.Players. You can do this:

part.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") ~= nil then
        local plr = game.Players:WaitForChild(hit.Parent.Name)
    end
end)

Also, at line 6, you wrote that the GUI named "Black" will be active after the player touches the part. But the activity of a Gui doesn't change its visibility. Instead you can set the "Visible" in the Gui named "Black" already false , then when the player touches it you make it to true:

part.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") ~= nil then
        local plr = game.Players:WaitForChild(hit.Parent.Name)
        plr.PlayerGui.Black.Frame.Visible = true
    end
end)

Also, you can simplify the transparency effect of the gui with loops:

part.Touched:Connect(function(hit)
    if hit.Parent:FindFirstChild("Humanoid") ~= nil then
        local plr = game.Players:WaitForChild(hit.Parent.Name)
        plr.PlayerGui.Black.Frame.Visible = true
        for i = 0, 1, 0.1 do
            plr.PlayerGui.Black.Frame.BackgroundTransparency = i
            wait(0.1)
        end
        wait(2)
        for i = 0, 1, 0.1 do
            plr.PlayerGui.Black.Frame.TextLabel.TextTransparency = i
            wait(0.1)
        end
    end
end)

Hope this helped!