Sorry, that is backwards. When you climb with a blocked pitot, the airspeed winds up, not down. Static is dropping and pitot pressure (trapped) stays constant. Delta P is higher which indicates higher airspeed. If I hadn't wound an airspeed completely around once climbing on top of an overcast, I'd probably be confused also.
So you're assuming unequivocally that the DRAIN port was also blocked? Because most of what I can find relating to block pitots DOES NOT support that as the 'usual' failure mode. With the drain OPEN and the main port blocked, the pitot internal pressure drops to near static, resulting in decreasing speed indication. I can't find the reference to hand now, but in something like 90% of recent pitot icing incidents this was the failure mode - blocked main port and open drain.
Secondly, how does that tie into the BEA reports of the speeds dropping below 60kts and then recovering?