There is a famous story about the Baalei Tosafos using their own blood to write their pirush on the chapter Merubah in Bava Kamma before they were massacred. Is there a source for this?
According to this link: According to the Minchas Elazar of Munkacs (Divrei Torah, eighth edition, 31), his grandfather, the Bnei Yissaschar, had a tradition that the greatest Baalei Tosafos (most of whom were killed al kiddush Hashem) wrote their long tosafos on perek Meruba of Bava Kamma the night before being called to judgment by their wicked enemies. They were threatened that if they refused to convert they would be killed. Apparently, this occurred after their first chiddushim were burned.
Note this does not mention writing with blood.