I am having an issue with par-baked bagels

First, I am located in Asia.  We have been running an artisan NY Deli for many years.  We are looking getting into the wholesale our bagels.  We have tested our par-baked bagels.  The way we do it so far is we make them like usual but we only bake the 80% after boiling them, let them cool off comply and then blast freeze them.  We try a bagel a week.  They are stored in a normal freezer.  We take one out the night before and let it thaw in the refrigerator.  We then bake them the last 20% the next morning.  We have run into two issue.  After a month in the freezer the dough starts to discolor a bit and only around the inner hole.  You do not see the discoloring from the outside only when you cut the bagel in half.  The coloring is a dark gray.  The taste is fine but visually is is not appealing.  Also the bagels are a bit dry.

What maybe making the discoloring issue?  How can I have a moister bagel when par-baking them.

That you in advance for your assistance!

Matt