Clearly the door edge has not been sealed with varnish. It has bleached to some extent and the black staining may well be due to mould (or dirt.)
You said in another post:
"However, my concern is if they build during winter is there a higher chance of dampness of wood during build phase?"
No Oak I am aware of has black in the grain.
It can easily be rubbed down and varnished.
The fact that the house builders don't want to means you should name and shame them so others are warned.
Be sure to give them negative responses in the 8-week survey you will be sent, especially a 'NO' to Q1.