Functional divergence after gene duplication plays a central role in plant evolution. Among cereals, only barley, wheat and rye accumulate delphinidin-derived (blue) anthocyanins in the aleurone layer of grains, but not rice, maize and sorghum. The underlying genetic basis for this natural occurence remains elusive. Here, we mapped the barley Blx1 locus involved in blue aleruone to a ~ 1.13 Mb genetic interval on chromosome 4HL, thus identifying a trigenic cluster named MbHF35 (containing HvMYB4H, HvMYC4H and HvF35H). Sequence and expression data supported the role of these genes in conferring blue-coloured (blue aleurone) grains. Synteny analyses across monocot species showed that MbHF35 has only evolved within distinct Triticeae lineages, as a result of dispersed gene duplication. Phylogeny analyses revealed a shared evolution pattern for MbHF35 in Triticeae, suggesting that these genes have co-evolved together. We also identified a Pooideae-specific flavonoid 3',5'-hydroxylase (F3'5'H) lineage, termed here Mo_F35H2 which has higher amino acid similarity with eudicot F3'5'Hs, demonstrating a scenario of convergent evolution. Indeed, selection tests identified 13 amino acid residues in Mo_F35H2 which underwent positive selection, possibly driven by protein thermostablility selection. Furthermore, by inquiring the barley germplasm there's evidence that HvMYB4H and HvMYC4H have underwent human selection. Collectively, our study favours blue aleurone as a recently evolved trait resulting from environmental adaptation. Our findings provide an evolutionary explanation for the absence of blue anthocyanins in other cereals and highlight the importance of gene functional divergence for plant diversity and environmental adaptation.