Why do all my fuses appear burnt?

My radio has been going off and on intermittently. I figured maybe it was the fuse that needed to be replaced. I don’t have a multimeter. I removed the fuse and it looked burnt so I just replaced it. I pulled a few others out and the same thing. I know that when you look at the fuse you should be able to see where it’s disconnected in the middle, if it’s blown. However, all of mine appear fine at that point but look burnt. Is this normal? Should I just replace all of them?

They are plastic. Elevated under-hood temperatures, petroleum vapors, atmospheric dust and contamination, and just aging can discolor plastic.

For the cost of a box auto of fuses you could have a basic DMM and simply check them - And a lot of other things.