The het's are quite black appearing but I figure that they will lighten up to look like their mother. Do you agree?
The het brooksi do lighten up. Way back when (70's-90's) there were no hypos or lavenders. Enthusiasts had to do things the old fashion way... pick out the best "normal" from a clutch and hope for the best. This is not as easy as it sounds. One required a good eye and LOTS of experience with brooksi because the difference from a normal baby turning into something spectacular was hard to tell when the snakes were neonates. One of the key ways to finding one was go to a reptile show and look at several breeders who produce normal brooksi and look through thier offspring offered for sale. Then buy them, raise them up and hope for the best. As a general rule all brooksi lighten up except some of the more "floridana" looking phases but even they lighten up to some degree.
Today the hypo brooksi kinda ruined the whole search for the perfect yellow brooksi because the hypo gene has eliminated all or most of the black creating the ULTIMATE yellow brooksi. Anyone with $50 can get one of these today and no searching or experience is neccessary.
Finding a real light yellow looking "brooksi" is still hard to come by and most don't have the roots to understand the value in finding one of these.
I have a dark phase female Brooks that is the color that she is now.
That would be an impossibility. The female you are comparing your het to is a reccessive gene. But if your het is very light it will lighten up even further.