Incidentally, I don't know Latin, but I think the tagline is a better rendition of our --- what do you want to call it --- slogan, motto, or epitaph. The grammar may still be a little dicey.
JP II cited it as follows:
Sanguinis martyrum, semen christianorum
http://www.vatican.va/jubilee_2000/magazine/documents/ju_mag_01031997_p-56_en.html
That said, infalibility doesn’t extend to grammar. The omitting the est is no problem, but the genitive plural still looks a bit dicey to me. My search also came up with alternative spellings of the genitive. I have four co-workers with better latin, so if no one resolves it, freep-mail me and I’ll ask around work.