blkid.h typedefs as follows:
typedef struct blkid_struct_dev *blkid_dev;
That is the only appearance of the token blkid_struct_dev in blkid.h.
grep -rl blkid_struct_dev /usr/include
returns only /usr/include/blkid/blkid.h. So I'm assuming the struct is defined somewhere in a -dev package I don't have installed (except it doesn't appear anywhere else on my Slackware box, either, and Slackware includes all the headers with its packages).
Where is this struct defined?