I call this form Ryūketsu 「流血」. In a touch of serendipity, it actually means 'bloodshed'.
I personally think Ryuuketsu（流血） should be more of the ultimate form since apart from the meaning of 'bloodshed' it would also work in a way as a portmanteau for Ryuuko（流子） and Senketsu's（鮮血） names, hinting that it should be something like a perfected synchronization, not a berserking form.
'Bloodshed' may sound violent and all but yeah given that the basic form is already called 'fresh blood' to begin with, so I think you get the idea.
So this form could be Shaketsu〔瀉血、Bloodletting）, Konketsu（混血、Half-blood), or just Senketsu II（潜血ＩＩ、Occult-blood) with the different kanji
