Take a look at the photo below. The bamboo on the left side is natural - that is, it hasn't been cooked or touched with any flame or heat. We refer to this as "blonde" bamboo. The right side of the bamboo piece has been flamed with a torch. (there's also a leaf node in the middle - a natural imperfection which shouldn't be used in the rod)
Today it's fairly typical for rod makers to give at least some flame treatment to their bamboo &/or to cook it inside an oven, or maybe both. Both of these (cooking & flaming) are done to temper the bamboo. Cooking the bamboo in the oven is a slower process, the flame method is quicker. The oven - usually - doesn't darken the bamboo as much either.
Why do we do this? Well, the theory is it's done to drive out excess moisture from the bamboo, thus making it more resilient & spring-like, giving it more bending strength. I should mention that the verdict is still out on this. There's no real hard science either way about tempering bamboo used in a fly rod & every rod maker does it a little differently, based on their experiences of what has worked for them in the past. There was a time when rod makers didn't put any heat on their bamboo. Those were the days when all rods were blonde rods.
Some time around 1914, the famous & legendary rod maker Eustis (E.W.) Edwards discovered that by exposing the bamboo to an open flame he could both temper & color his bamboo. The darker colored rods quickly caught favor with anglers & soon many rod companies were darkening their bamboo.
Some even took it to extremes, making their bamboo rods very dark - almost a chocolate color. This is accomplished by cooking the bamboo in an oven with some very caustic chemicals. The fumes of the chemicals increases the darkness of the bamboo, but the fumes can be very dangerous & harmful to breathe.
These darker, flamed rods led to new aesthetics. Rods became what some consider to be more elegant, with subtle colors of wraps & blued metal. Blonde rods started to fall out of favor with both anglers & rod makers. It didn't hurt that rod makers found that flaming the bamboo also helped to hide small cosmetic imperfections in the blonde bamboo as well.
Personally I don't like to make my bamboo too dark - & certainly no caustic chemicals for me, thank you very much. I like a light to medium honey colored rod. I think this gives the bamboo enough tempering & it darkens it just enough to allow the color of the rod shafts to harmonize with just about any color of silk thread wrap.
As they say, beauty is in the eye of the beholder. Some of you will want a flamed rod, while some prefer a lighter colored rod. I like them all. Ever since E.W. Edwards first put flame to bamboo it's become a regular "look" in fly rods. Maybe someday blonde rods will become the rage again & the trends will come full circle?