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why dont you just draw a really big black rectangular box in paint? then when you put it in your game, make the transparency what you want it to be.
(https://rmrk.net/proxy.php?request=http%3A%2F%2Fmembers.shaw.ca%2Fascen%2Fgradient.png&hash=72e09c8d6094d82868e8db699ed372787cc9a0aa)
Credit to blizzard I assume?