I guess they meant the "pureness" of Grimm blood that comes from within. Marie and Kelly may have been tangible ghost that helped him fight but Nick had to dig deep within himself, summoning the strength within himself from his ancestors since he carried their blood. I think it's because Grimms are supposed to be viewed as doing a sort of holy work in fighting evil so the bigger the foe, the more they powerful they get, not because they alone are doing the fighting but that they are mystically "powered up" should the situation call for it.
The mixing of Grimm, hexenbiest and wesen blood created a deadly potion than no one/thing could withstand but Zerstörer is supposedly evil incarnate, and is above such "tricks" If you will. The Grimm's strength of one's blood could be similar to Charmed "power of three" incantation that pretty much saved the day for the three witches.
The mixing of Grimm, hexenbiest and wesen blood created a deadly potion than no one/thing could withstand but Zerstörer is supposedly evil incarnate, and is above such "tricks" If you will. The Grimm's strength of one's blood could be similar to Charmed "power of three" incantation that pretty much saved the day for the three witches.