and why Isawa used blood offerings but they cant now.

Kansen are basically corrupted Kami. Rather than acting as part of the natural world, they serve the will of Jigoku and seek to spread the taint.

As far as blood magic, why it became corrupted has to do with the First Day of Thunder and Isawa's used of blood magic to bind the fallen Kami Fu Leng. My guess is that blood magic at one time represented a connection to one's spiritual power, and that channel or conduit was was perverted when blood magic came into contact with the power of Fu Leng and Jigoku.

If I recall my Lore correctly, there was a little bit about the Gods that protected Isawa’s tribe protecting them from the corruption of blood magic, but with the influence of Jigoku and the amount of practitioners of blood magic increasing, the Gods of Isawa’s Tribe couldn’t protect everyone anymore so eventually they changed to this new format of making other offerings.

Just a point of clarification, the above-mentioned gods of the Tribe of Isawa are the Seven Fortunes. Basically when Amaterasu was persuaded to accept them as part of the official pantheon, their duties extended to all of Rokugan, causing the (also above-mentioned) issue of not being able to protect everyone under their care from becoming tainted through maho. It probably doesn't help that anyone can interact with kansen through maho, so the Fortunes would have to protect the entire peasant population as well, not just the shugenja, or even just the samurai.

3 hours ago, Kaito Kikaze said:

Just a point of clarification, the above-mentioned gods of the Tribe of Isawa are the Seven Fortunes. Basically when Amaterasu was persuaded to accept them as part of the official pantheon, their duties extended to all of Rokugan, causing the (also above-mentioned) issue of not being able to protect everyone under their care from becoming tainted through maho. It probably doesn't help that anyone can interact with kansen through maho, so the Fortunes would have to protect the entire peasant population as well, not just the shugenja, or even just the samurai.

Thank you that makes a lot of sense. so are you refering to when the Hantei combined shinseiism, and Fortunism?

3 hours ago, Daeglan said:

Thank you that makes a lot of sense. so are you refering to when the Hantei combined shinseiism, and Fortunism?

Had to look that up, honestly. It looks like the event you're referring to was an edict from Hantei Genji. This appears to predate that, as from the wiki it says Shiba himself asked his brother Hantei to persuade their mother Amaterasu to accept the Seven Fortunes into the pantheon and the Isawa as allies. Interestingly enough, from how this is summarized it seems like the Seven Fortunes were more or less unique to the Tribe of Isawa prior to that.

The official canonical explanation (in the AEG L5r) is in a really small part of Way of the Shadowlands, it's that the Pit of Fu-leng, when it opened, released kansens and corrupted local kamis, which now respond to the use of blood magic. The explanation about the Seven Fortunes is an in-universe explanation for the problem.