if you are trying to make ralationship links between post types, the easiest way is to use the great “posts 2 posts” plugin by Scribu :
http://wordpress.org/extend/plugins/posts-to-posts/
here is the documentation :
https://github.com/scribu/wp-posts-to-posts/wiki
hope that could help.
seb.
EDIT :
Is there a way to list all the posts created under one custom post
type (eg: Blueprints) in the edit screen of another (eg: Buildings) in
a checklist (or similar)
yes ! in the edit screen of each post types depending on the associations you create between them. not a checklist, but similar, well, better ! this plugin gives you the ability to add/remove/reorder, with drag and drop, your linked posts/post types/pages and even users if you need…
so that individual bluebrints can be associated with individual
buildings?
absolutely ! not only “one to one” relationships inbetween different post types, but also “many to many” and “one to many”… you choose. see the manual.