This ageing and carbonization is the only reason an old $2 industry fuse should be replaced, because it’s seen too many switch-on surges.
And a $100 boutique will be the same after it sees too many switch-on surges also.
A slow-blo fuse "ageing" right to left
https://peakpinball.com/wp-content/uploads/2013/01/fuse-8312-640x426.jpg
A fast-blo fuse "ageing" left to right
https://i.stack.imgur.com/0uqWX.jpg%20
Cheers George