Roblox UI Blur

local lighting = game:GetService("Lighting")

local blur = lighting:FindFirstChildOfClass("BlurEffect")
local blurSize -- variable to store the default blur value
local open = false -- variable to store whether the GUI is open or not

local function setup()
    if(blur) then
        blurSize = blur.Size -- store the default blur value if instance "Blur" already exists
    else
        blur = Instance.new("BlurEffect")
        blur.Parent = lighting
        blur.Size = 0
    end
end

local function openUI()
    blur.Size = 15 -- Any value of your choice, I'm using 15
    open = true
end

local function closeUI()
    if(blurSize) then -- if we have a default blur value
        blur.Size = blurSize
    else
        blur.Size = 0 -- we keep the blur effect, but give it size 0
    end

    open = false
end

setup() -- we run the setup function to store the default value, and whatever else you want;

--[[
Usage example:

button.MouseButton1Click:Connect(function()
    if(open) then
        closeUI()
    else
        openUI()
    end
end)
]]