Bluing is a chemical process that darkens certain metals, turning them from their natural shinny appearances to a darker color. It's a common treatment to many firearms & some fishing rods. The idea behind this is that by darkening the metal, it won't reflect the light as much & won't give away your location &/or scare your prey while you're afield hunting or fishing. Bluing is a surface treatment only. Any surfaces that are blued must be coated with a protecting layer of either varnish or clear coat, or the bluing will quickly wear off. The darkness of the metal can be somewhat controlled by the person bluing it, making it lighter or darker. It's important to know that bluing doesn't make the metal exactly black, but a dark shade of gray. Perhaps the best way to describe its color would be "gun metal gray".
On fly rods, bluing is typically considered only with bamboo rods, but it can be done on some synthetic rods as well. Traditionally, rods with darkened metal are made from bamboo that's been flame treated more, giving the cane a darker look. That is to say, it would look a little odd to see a blonde rod with blued metal. However there are exceptions to every rule & I have seen blonde rods with only their ferrules blued.
So what all gets blued on a fly rod? Typically the darkened parts are:
-winding check (metal ring just above the grip)
-metal frame of an agate stripping guide
-cap & ring of the reel seat
Remember that the idea is to reduce your visibility to the fish by not having any shiny metal flash in the sun & scare away the fish; & also remember that, based on the traditions of bamboo fly rods, there are always exceptions to the rules (more on that later).
What about the line guides? Well bluing is a surface treatment & any bluing will quickly wear away that isn't protected by a clear coat. If the line guides were to be blued in the normal way, the bluing in the guides where the line comes in contact with them would be gone in no time flat, even with a protective coating. So today rod makers use either black line guides that have been blackened by a commercial process (much tougher & harder than normal bluing), or guides made of titanium.
Okay, but I haven't mentioned the screw-locking style reel seats. Here is the big exception to the rules. When the screw-locking reel seat was invented there was no process available that would allow the threads to be blued without the bluing quickly wearing off. So the fly rods with the early screw-locking reel seats that had their other metal parts darkened left the metal of the reel seat "natural" & not blued. This became the tradition & standard of bamboo fly rods....but only when the rod had a screw-locking style reel seat.
Today it's perfectly normal & acceptable to see a rod with blued ferrules, etc but a natural, non-blued screw-locking reel seat. However, if you let tradition set your rules, whenever you see a rod with a cap & ring style seat, the cap & ring are blued to match the other metal on the rod. In fact, it would be odd to see a bamboo fly rod with a darkened screw-locking reel seat.
But you might have seen them. It is possible to darken the threads on a screw-locking reel seat. However, it can only be done by an expensive commercial process. In fact, I don't believe these seats are available anymore as the only company I know of that offered them went out of business. So basically, forget about them. The practical justification for not bluing the screw-locking reel seat metal was that the reel seat is under your wrist or forearm when fishing anyway, so it really can't be seen. Well.....okay?
I think the reason most anglers choose to have rod with blued metal is because of how it looks. That's fine. Many rods with blued metal have a classy, distinguished look about them. I've fished with both blued & non-blued rods & I can't honestly say that it helped me or hurt the number of fish I fooled (or lack there of). No, I suspect better casting & fly presentations would have probably helped me more.