The printer told you turn overprint OFF for black type? Run.
Lots of poorly trained prepress techs have their RIPs set to ignore overprints by default, and the knockout could only have happened if the applied overprint was ignored.
Peter is right. InDesign overprints Black by default. If you want to avoid overprinting black you have to create your own black swatch. But normal text is always overprinted if black, only huge sizes are knocked out.
FWIW: The Illustrator overprint mode controls if process colours can overprint or not. If it is set to 0 overprinting only happens with spot colours.
In other words: Black text over yellow background needs the OPM set to 1.
Which it is by default when you export from indesign.
Overprinting is not the same as "mutliply". Meaning: If your text as a yellow value of anything but 0, that value will be used, which can also result in trapping problems.
Therefore another explanation for your print result would be some colour management gone wrong, i.e. they somehow converted your pure black to a black with some value in the yellow channel.
The recommendation to turn overprint OFF is quite mad though.
Thank you everyone.
So in short my file was supplied correctly. I thought this was the case but I didn't want to be too arrogant and wanted to check. I checked the file supplied and I had definitely supplied the text as 100K
Let's hope they refund or reprint my job.
Just to update you - the printers have absolutely insisted that I must turn overprint off in order to have my file correctly reprinted (!!??) and also suggesting an alternative that I set my 8pt, script black text to rich black (again, !!??)
I don't understand it as they've printed so many jobs correctly before.
They are absolutely convinced I'm wrong and I don't know how to persuade them, but I'm guessing someone in the studio somehow turned overprint OFF and therefore a slight misregistration has caused a white outline.