X: Wirst du mir helfen?
Y: Blas zum Antritt an!
I suppose this expression boils down to an enthusiastic "yes!", but I'm at a loss as to how to interpret its meaning.
The best english translation I can think of is
to sound the rally
IOW, someone blows the horn to make people/soldiers collect for the appeal. Or, alternatively:
roll call
Right, this means
I'm ready to help! Now or later, tell me when and I'll be there!
You won't hear »Blas zum Antritt an!« on the street, sounds like hunter speech, a command to start a hunt [for wild boars]. Colloquially we would say e. g.
Sag mir wann! (Tell me when!)
or
Jederzeit! (Any time!)
Keywords:
anblasen = durch ein Horn, Signal o. Ä. das Zeichen für den Beginn von etwas geben; etwas ankündigen = give the starting sign for something by using a horn or something similar; to announce something
Antritt = der Beginn, das Antreten = the beginning, the start
So a more literally translation for »Blas zum Antritt an!« could be
Play the horn to mark the beginning (of my help)!
(I will help as soon as I can hear the horn!)
As has already been explained, blasen in this sentence refers to signalling something using a horn. I would like to add that I think that the usage in the example is wrong, though. There is zu etwas (Dativ) blasen and also (less frequent, I think) etwas (Akkusativ) anblasen. I do not think that there is zu etwas anblasen. So your correspondent should have answered:
Blas zum Antritt!