The “Blue Lady” covers are the original U.S. first edition covers for the “In Death” Series. They were titled “Blue Lady” covers, because each cover featured an image of a woman with blue skin.
The first seven books were printed with “Blue Lady” covers and then the “Blue Lady” covers were scrapped in favor of the “New York Landmark” covers, which depicted a different historical or significant New York Landmark (or just a random New York scene) on each new cover. When the original seven “Blue Lady” titles were re-issued, they were re-issued with a “New York Landmark” cover.
Many serious collectors of the “In Death” Series regard the “Blue Lady” covers as highly collectable, and a must-have if you want to own the complete series of books.