Monster Blood and Beware of the Purple Peanut Butter take place in the same universe.
However, nobody seems to notice that the chocolate cake and Monster Blood have similar effects once consumed. Effy says "growth spice" is what powers the cake, so who's to say that couldn't also be what's used in Monster Blood?
Maybe Monster Blood always contains growth spice, and Kathryn's spell in the first book somehow activated it? (Does Monster Blood actually start growing in any of the sequels?)
Yeah. I'm bored in case you couldn't tell.