Yes, a blued gun does need to be reblued after being engraved.
There is a very, very wide difference in price and quality among engravers. Inexpensive is about $500 and expensive runs $10K and up. There is a lot inbetween. Some engravers use only hand tools and others electric. As with any signifcant purchase, you should do your research first. There are a couple of engraving forums, and also an association of engravers website. These are useful to look at pictures, and to learn.
And something you need to accept, going in, is that you are unlikely to recover the cost of the engraving should you decide to sell the gun. For engraving to make sense for you, it needs to be something that you value and that you understand well.
I recommend buy Tom Turpin's Custom Firearms Engraving
, which has pictures of the work of various artists, but the best info is probably on the internet.
That said, I only know a little. It is an art form that people spend their lives learning how to do, and in studying how to appreciate it.
This is by Dan Love: