I have built many fly rods and have never had any problem because I use materials made for what I am building. One acception is building a fly rod from a noodle rod blank. I have not done this yet, but have seen some nice rods used with fly reel and line for fishing salmon, steelhead, etc.

One thing to keep in mind, I believe you could use any blank if you remember the spine positioning of each type of rod. The fly rod guides are along the spine of the rod and pointed downward, spinning rod's guides are located on the opposite side of the spine and also pointed down with the spine upwards. Bait casting rod's guides are located on the spine BUT the guides are pointed up when holding the rod in your hand. Thus, they have more power for larger fish (that is why when fighting a large fish a fly fisherman will turn their rod over for strength).

For finding a spine see the other post on this BB. I prefer bending the rod blank while rolling it on a table top. This allows me to "feel" the "bump". A spine tool like the one in my local fly shop is better and most of the time I have the blanks spined and marked before I leave the shop.

Take everyones advice and build a rod.