We first need to know:
Which kind of PDF (X/1a, X4 etc.);
Blend color space setting of your file;
Used icc profiles for PDF (in case of X/1a);
The color space of the placed PS file (RGB?).
I've selected: ADOBE PDF Press Quality
Compatibility Acrobat 5 (PDF 1.4)
Transparency Blend Colour space is RGB
CYMK images are in the document, there may be some RGB images as well.
Again, I have had zero issue before. If I try to print the doc, the print preview shows perfect black.
I also created a fresh Indesign document with a single Black Box. The black box when made into a PDF changes to grey/brown.
Some first things first: as memory serves me well, Press Quality (never use that, go for a PDF X standard!) is not color managed, so RGB black will stay RGB and CMYK black will stay CMYK which of course means different blacks. Also if you do color manage to CMYK that will result in different blacks, 100% K verses process black of CMYK values. This you can fix by the way in Acrobat Pro afterwards.
Second: for print your blend space should be CMYK in most cases and is the standard when you choose a new document for print, what is the reason you set it to RGB?
Third: what are your settings for handeling black in the preferences (appearence of black)?
I see this is a duplicte post that is answered in the copy subject. Please close this.