The basal petal blotch of Paeonia rockii is the primary genetic source of blotch traits in cultivated tree peonies. To clarify the timing of blotch initiation，its expansion pattern，and the relationship between blotch development and pigment accumulation，basal blotch formation at seven developmental stages were systematically examined in ten P.rockii cultivars. At stage S2，single pigmented cells first appeared and gradually developed into pigment cell clusters，consisting of a deeply pigmented central cell surrounded by lightly pigmented peripheral cells. Blotch expansion occurred through the proliferation and enlargement of these pigment cell clusters. The blotch outline was initially established at stage S3，preceding the visible coloration of the petal background，and a second peak of pigment cell proliferation was observed at stages S6-S7. During S2-S3，significant decreases in L* and b* values and an increase in a* value were recorded in the blotch region. Anthocyanins were detectable in the blotch as early as S2，with cyanidin-3，5-di-O-glucoside（Cy3G5G） and peonidin-3，5-di-O-glucoside（Pn3G5G） as the predominant components. Linear regression analysis indicated that Pn3G5G had strong explanatory power for changes in L*，a*，and b* values and was the key anthocyanin contributing to the formation of purple-red blotches. Pelargonidin-3，5-di-O-glucoside（Pg3G5G） may be involved in the formation of black blotches. In summary，blotch formation in P. rockii is a dynamic developmental process characterized structurally by the expansion of pigment cell clusters and biochemically by the spatiotemporally specific accumulation of anthocyanins.