I have checked the engraveable/not engraveable gun list and didn't find any mention about blued Ruger vaqueros. Do they have the same hardness problems with the loading gate and cylinder as the stainless Rugers?
As Beathard said, they are engravable. However, not all of them come with aluminum backstraps. Both of mine came with steel. The ejector rod housing is usually aluminum, though. You can get a steel replacement from Brownell's.