score:2
It seems that you have a document according to § 81 Abs. 4 AufenthG (Fortgeltungsfiktion)
If a foreigner applies for an extension of his or her residence title or for a different residence title before his or her current residence title expires, the current residence title shall be deemed to remain in force from the time of its expiry until the time of the decision by the foreigners authority.
This document should be an official form with a sticker on it (which is not placed in your passport). On page 3 the third box with this text needs to be ticked:
[Bis zur Entscheidung der Ausländerbehörder gilt:] der Aufenthaltstitel als fortbestehend (§ 81 Abs. 4 AufenthG)
This is how such a document should look like:
If that is the case, your residence title does not expire if you only temporarily leave the federal republic (as regulated in § 51 AufenthG) as long as the document is still valid as indicated by the sticker.
This is also explicitly stated in 81.5.3 VwV-AufenthG (which is the relevant administrative regulation):
Die nach Absatz 4 ausgestellten Fiktionsbescheinigungen berechtigen anders als die nach Absatz 3 ausgestellten Bescheinigungen in Verbindung mit einem anerkannten und gültigen Pass oder Passersatz zur Einreise in das Bundesgebiet und nach Artikel 21 SDÜ zu Reisen innerhalb des Schengen-Raums. Voraussetzung ist neben der Verwendung des durch die AufenthV vorgeschriebenen amtlichen Vordrucks, dass auf Seite 3 des Trägervordrucks das dritte Ankreuzfeld angekreuzt ist. Ein- und Ausreisekontrollstempel sind im Pass oder Passersatz, nicht aber auf der Fiktionsbescheinigung anzubringen.
(Which just says that as long as the above mentioned box is ticked and the document is still valid you can enter the Schengen area and border stamps should be placed in your passport.)
To enter the Schengen area present both your passport (which contains the old visa) and the valid Fiktionsbescheinigung at the border.
The document is also mentioned in the Schengen list of residence permits issued by member states, so even a non-German Schengen border guard should know the document and let you pass:
Fiktionsbescheinigung (Provisional residence document)
Only if the third box on page 3 is ticked – the residence permit continues to be valid. Entry is allowed only in connection with an expired residence permit or visa.
If the first or second box is ticked, the “Fiktionsbescheinigung” does not entail the right of entry without a visa.
The Fiktionsbescheinigung could theoretically have a restriction on leaving the country which would be clearly mentioned on it but I don't think this is ever done.
As soon as the decision on your Blue Card is made, the Fiktionsbescheinigung automatically becomes invalid. If the decision was negative, obviously you can't enter again.
In any case, you should talk to your Ausländerbehörde before you leave Germany and not take advice from random stranger on the internet.