Final answer:
Jim calls Laura 'Blue Roses' because he mistook her bout with pleurosis for 'blue roses,' which symbolizes the unique and rare beauty he sees in her.
Step-by-step explanation:
Jim calls Laura "Blue Roses" because he misheard her when she told him about her bout with pleurosis. He thought she said "blue roses" instead of "pleurosis." This nickname is indicative of the special friendship and remembrance he holds for Laura, and it captures the delicate and unique beauty he perceives in her, much like that of a blue rose which is rare and not found in nature.