Although some species that accumulate only cyanidin (Cy) in nature can produce blue flowers through iron ions, there has been no evidence of blue chrysanthemums being generated in this manner. This study revealed that flavonoid extracts from the ray florets of the chrysanthemum cultivar ‘Wandai Fengguang’ turned blue when exposed to Fe³⁺. Samples that could turn blue were labeled as CB (Cy-determined blue flowers), while samples that did not turn blue were labeled as CN (Cy-determined non-blue flowers). After a series of experiments, a stable screening system was established using flavonoid extracts containing NaAc buffer at pH 5.5 and a total anthocyanin concentration (TAC) of 30 μmol·L⁻¹, and the addition of Fe³⁺ from 0 to 0.25 μmol·L⁻¹ allowed for the selection of five CB samples from 39 chrysanthemum cultivars. All five CB samples exhibited flower color phenotypes that belonged to Cluster-I with redness (a*) values ranging from 29.03 to 45.99, yellowness (b*) values from −11.31 to 3.77, and brightness (L*) values from 29.07 to 45.99. Additionally, the ratio of TAC to total luteolin concentration (TLC) was found to be a critical factor for distinguishing between CB and CN samples. To realize the desired blue hue in the flavonoid extracts with the participation of Fe³⁺, a TAC to TLC ratio of 2.25 and above is required. Moreover, the protoplasts and ray florets of CB samples that turned blue with the involvement of Fe²⁺ showed great potential for cultivating blue chrysanthemums through ferric–anthocyanin chelate. Overall, this study reveals that blue flowers can be cultivated through the increase in the iron ion concentration, combined with the accumulation of Cy.