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According to Β§51 AufenthG your Blue Card expires in the following cases:
6. if the foreigner leaves the federal territory for a reason which is not temporary in nature,
7. if the foreigner leaves the federal territory and does not re-enter the federal territory within six months or within a longer period set by the foreigners authority,
Even though six months have not yet passed since you left the country, leaving for work in another country will most likely be considered a non-temporary departure.
If you have not notified AuslΓ€nderbehΓΆrde, it is likely that they do not know the reason for your departure. However, even if you are allowed to go through the border, your residence permit has legally expired at the moment when you left to the UK.