From what I gathered from the story, I thought along the same lines as this person.
>>So, speculation follows: the black hole that is destroying the Old Sun is
>>merely the necessary complement to the White Fountain. One has caused the
>>other, and are both different sides of the same coin that is Severian.
>
>While I've been aware that there was a black hole in the Old Sun, and that
>the catalyst for the conversion of the Old Sun to New Sun is a White,I've
>never been able to imagine what exactly is supposed to happen when White
>Fountain meets Old Sun and why this event creates a young regenerated sun.
>It's a bit like the two scientists at the blackboard in Larson's cartoon:
>"and then a miracle happens".
>
>So to be explicit: It seems plausible that the black hole in the Old Sun is
>*directly* connected to the White Fountain. Energy/matter exiting through
>the black hole in the Old Sun appears from the mouth of the White Fountain.
>Which means that its not a Black Hole complete with singularity but instead
>both phenomena represent the two mouths of the same wormhole. This has some
>nice cyclical resonances if we can get over the dualist implications it has
>for Severian's character and nature, and also gives us a way to think about
>the rejuvination of the Old Sun.
>
>Kieran C.
I got that from this site which kicks ass.
http://www.urth.net/urth/archives/v0007/index.shtml
