Here it is:Interlude 7 Baxil
Here's another relevant quote from the same chapter:
I have a semi-explanation for this: remember that Shallan's essence is almost certainly blood and that her creative skills involves taking/capturing a memory.
I'm not sure how to extend that but it may be that the rational lies that way->Dallinar, overcome with grief after his wife's death goes to Nightwatcher to have her memories removed. The Memory, falling under Shash, means that her curse is that of blood.
You can make the same reasoning for Av's father: His love for his family caused him to seek out Nightwatcher's help, pinning him as loving. As a result, his curse was that of the eyes.