can be seen in blahtexml-0.9-Makefile.patch
PKG_CONFIG ?= pkg-config
These direct calls seems to be quite common, if I grep through the patches available in the tree.
Maybe it might be worth to create a tracker bug and add all other packages with direct calls to pkg-config there, if this is a problem for you?
Feel free to fix it yourself, as I might not have the time in the next days.
Yes, others do it wrong as well.
+ 26 Dec 2013; Julian Ospald <firstname.lastname@example.org> blahtexml-0.9.ebuild,
+ respect PKG_CONFIG wrt #478514, add ~x86