Transport Briefing has a good summary of the developments taking place at Blackfriars Station:
"...Balfour Beatty will rebuild and extend the existing station to span the Thames with a new entrance on the South Bank. The existing bridge will be reassembled in two halves with the spans replaced one-by-one with wider pieces to allow the longer platforms and erection of a roof across the station."
Meanwhile, Regen.net takes a wider view of the project and its implications. Also worth a read.