hi typomaxx
where are this illustrations from?
if you build them in scribus, you have to select the pure 'black' (0%,0%,0%,100%) – depending on your color set, there are many different shades of 'black'.
if the illustrations are generated in another application, you should check the colors before the import.
acording to the specifications of pdf/x-3, scribus doesn't convert the colors while generating the pdf…
utnik
edit: how are the grays in your illustrations buildt? (cmy, cmyk, k only, rgb…)