This is in reference to a 'blue movie', a euphemistic term for a pornographic film.
It has since been toned down somewhat and phrases like a bit of blue can be used to say that something is 'adult' in nature. Often this includes dealing with sexual material but doesn't necessarily mean visually pornographic; a comedian who tells jokes with a sexual theme could be called 'a bit blue'.
going a bit blue then would mean that the programme/character in question is normally 'clean' but has begun to include more adult themes.
It would appear that the term has been used thus since the early 1800s, originating in Scotland, though a clear connection between the colour and the connotation has not been settled on.
N.B.: Bolton comedian Peter Kay is famous in England and well known for his family friendly stand up material. In his shows he will occasionally tell a joke with some sort of subtle sexual reference or other 'adult' theme and then follow the joke by saying 'a bit of blue for the dads there'.
I don't speak German, but from the examples you've given I would say that the English and German usages are similar, but not identical. In English, it's almost always a rebuke, rarely a warning. It refers to forgetting about social rules, and your place within them. The expression can be mild or strong depending on the tone of voice, the speaker and the context.
In your examples:
John, we are going out to dinner tonight. Please don't forget yourself.
A mild rebuke. John has behaved badly in the past, and the speaker is gently requesting that he control himself tonight. Depending on the tone of voice, this could be a desperate plea from someone who feels slightly inferior to John (and therefore doesn't want to use stronger language) or a patronising order from someone who feels superior to John to the extent of treating him like a child.
I believe you forget yourself sir!
A strong rebuke from a social equal or superior. The speaker is very angry, and yet is trying to stay within the social rules himself.
I'm forgetting myself. I haven't offered you a drink yet!
The speaker has caught himself out in a social error, and is at once acknowledging his mistake and apologising for it. It's a mild self-rebuke, although tone of voice and context will show how embarrassed the speaker feels over the faux pas.
As you can see, it's all about context. The expression is almost never used in isolation, so the context is usually plain. The third one is probably the most common, though.
Finally, can this expression used as a threat or warning as in German?
It can, but it's not common. In English, such a threat would usually be wrapped up in a few more words. ("If you don't stop that, I might have to forget I'm a gentleman.") Like the second example, above, the speaker is trying to stay within the rules himself, while making it clear that he finds the other person's behaviour unacceptable. However, as social rules erode, this usage is becoming increasingly old-fashioned.
Best Answer
To "work out of somewhere" means that you are based somewhere. At one time it was usually used of someone whose work involved travelling. A salesman might "work out of New York", meaning they were based in New York, even though their actual time was mostly spent on the road (and therefore not in New York at all). His office-bound colleague would be said to "work in New York". In more recent times 'work out of' used as a synonym of "work in".
The phrase you ask about therefore strictly means that the job is based in Atlanta, that they will report to someone in Atlanta, and they will be paid in Atlanta. However in reality it probably means you need to be present in Atlanta. The phraseology might open the door a crack to the possibility of telecommuting or remote working.