Few weeks ago (approx 2 weeks before lock down) I applied for Blue Card at KeplerStraße, Berlin office and my pick up date is 27 April.
Am I going to get the Blue Card by Post at my address?
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This is really a matter that you should look up yourself.
Relavent links to their pages (mostly in English) have been listed below.
It is only there that you will find the latest information available.
The relevant point in the FAQ page seems to be:
I have an appointment to collect my electronic residency permit (eAT). Can I still come for this appointment?
If you do not yet have a residency permit, but have been given an appointment to collect your electronic residency permit (eAT), then please make contact with the office department responsible for you. Such enquiries will be considered on a case-by-case basis. You can find the necessary contact form here.
but this is something you should read through carefully.