Andersel, Before y'all get the official (read techie) answers, let me give ya my layman's version.
For me it is real simple. If I don't like the prop blur that came with the plane, I find one from my rather large inventory of planes and try it out. I base my choice on how it looks to me -- not anyone else. How dark, how reflective, how "complicated," number of blades, etc. Sometime one I really like on one plane will look screwy on another, so it's hunt and peck until I'm happy. I keep trying until I either fine one that suits me or someone on line offers up one they like. Banana Bos has created a huge number of alternate prop blur files over the years.
The actual process is also simple.
1...I select one texture folder in the target plane,
2...Rename the "prop_blur.bmp" to "prop_blur - ORIGINAL.bmp",
3...Copy in the new version,
4...Rename it using the original name,
5...Save the texture folder and,
6...Reload the plane to check the result
If I like the result, I create a folder under the plane called "Alternate BMPs" and stick in both the original and new version. This way I can stick in the modified version to any other variants I might add. This becomes a very handy folder as it allows me to include other alternative files I might wish to add to the plane such as different glass bmps, low or high reflective skin bmps, etc.
Calling the folder "Alternate BMPs" places it at the top of the list in the plane folder so regardless of how many texture folders I might have, it is always at the top of the list so I won't miss it.
Like I said, the simple solution
ps: You can create other "Alternate xxx" folders to keep other alternate things close to the plane like, sounds, panels, models, etc. that way all the relevant bits stay with the plane even if you take it offline and stow it in a hanger.