That looks like water related damage.
I was doing an inspection with a pest inspector one time and they had a shed that looked like it had had a fire on the inside (although it didn’t really seem like there had been a fire - it was years ago, so memory is a little fuzzy). He said it was fungus damage.
Something’s been repaired on the roof covering above…see the 4 strange nails in the 3rd photo. I’d surmise a small lightning strike that breached the shingles and water intruded for some period of time (hours, days, weeks) before repairs were made to the shingles and nothing was done to the framing.
Looks like good old rot to me. If there is a large amount of resins in the wood, they will often turn black like that from rot and staining. If you took a UV light up there, you will probably see that the wood has a lot of sap on the surface. The sugars break down and turn black. Often, you will also get mold growth on areas where the sap sugars have broken down. Take a UV light to it and if you see a slight yellow to green glow, you will have have a good indication.
Looks like water damage, Burn would look a little different , One question is the wood charred on the under side by the edge? I think there be more of a blister pattern if it was a lighten strike . Also the trail would be different.