Well sure. That is because it is changed everytime Gametooltip:Show() is called.
What you can do is sth like this:
Lua Code:
local backdropColor = {0,0,0}
local backdropBorderColor = {0,0,0}
local tooltipList = {GameTooltip, ItemRefTooltip, ShoppingTooltip1, ShoppingTooltip2, ShoppingTooltip3}
for index, frame in ipairs(tooltipList) do
frame:SetScript("OnShow", function(self)
self:SetBackdropColor(unpack(backdropColor))
self:SetBackdropBorderColor(unpack(backdropBorderColor))
end)
end