Blackjack and the other Daemonslayers prefer to keep the fact that he is Bloodbane a secret. This is because when people find out you were the evil tyrant that rose from the ashes of the Old Empire and enslaved the remains of a continent's human civilastion for a couple of centuries, awkward questions tend to arise (even though he's largely reformed).

As well as this, should the Emperor or the Arcanatoria (the high Council of Mages) discover he's still alive, they would spare no trouble to destroy him and any of his allies. Later on in the DS storyline Black's found out and this proves disastrous.