[Vlad]

You are already on the right track, spotting the fact the code is the same except in a very few locations. So fixing this shouldn't be hard. We first wrap this in a function, then we give that function an argument we call "parentFrame", then we can easily call the same code as many times as we like, and just provide it with a new frame to work with and do the exact operations.

local function SkinFrame(parentFrame)
	local bg = CreateFrame('Frame', nil, parentFrame)
	bg:SetPoint('TOPLEFT', parentFrame, 0, -26)
	bg:SetPoint('BOTTOMRIGHT', parentFrame, 0, 26)
	bg:SetBackdrop({
		bgFile = [[Interface/Buttons/WHITE8X8]],
		tiled = false,
		insets = {left = -3, right = -2, top = -3, bottom = -3}
	})
	bg:SetBackdropColor(0, 0, 0, 1)
	bg:SetFrameLevel(0)

	local trans = CreateFrame('Frame', nil, parentFrame)
	trans:SetBackdrop({
		bgFile = [[Interface\Tooltips\UI-Tooltip-Background]],
		tiled = false,
		insets = {left = -6, right = -5, top = -6, bottom = -6}
	})
	trans:SetPoint('TOPLEFT', parentFrame, 0, -26)
	trans:SetPoint('BOTTOMRIGHT', parentFrame, 0, 26)
	trans:SetFrameLevel(0)
	trans:SetFrameStrata('BACKGROUND')
	trans:SetBackdropColor(0, 0, 0, 0.6)
end

SkinFrame(PlayerFrame)
SkinFrame(TargetFrame)

There is a final form of optimization we can do, though not a big thing considering it's ONLY used two times. Larger addons, let's say a raid frame with 80 potential units on screen, we could look at the two tables used in bg:SetBackdrop() and trans:SetBackdrop() and considering it's the same table used each time, we could make it so we just reuse the same table from memory, since the function creates a new table each time it is called.

local BG_BACKDROP = {
	bgFile = [[Interface/Buttons/WHITE8X8]],
	tiled = false,
	insets = {left = -3, right = -2, top = -3, bottom = -3}
}

local TRANS_BACKDROP = {
	bgFile = [[Interface\Tooltips\UI-Tooltip-Background]],
	tiled = false,
	insets = {left = -6, right = -5, top = -6, bottom = -6}
}

local function SkinFrame(parentFrame)
	-- <snip>
	bg:SetBackdrop(BG_BACKDROP)
	-- <snip>
	trans:SetBackdrop(TRANS_BACKDROP)
	-- <snip>
end

myrroddin was quicker and replied while I was typing this out, but I'll hit the post button regardless.