What chemically happens when aluminum reacts with anthocyanins?

For example, in hydrangeas, when aluminum reacts with the anthocyanins in the flower's petals, the hydrangeas turn blue, but what chemical reaction occurs?
What chemically happens when aluminum reacts with anthocyanins?
"Two factors affect the color in Hydrangea macrophylla cultivars: soil acidity and the presence of aluminium in the soil. To obtain a blue hydrangea, aluminum must be present in the soil. The pH of the soil should be low (4 - 4.5). In alkaline soil aluminium is tied up. The plants are unable to absorb the existing aluminium and the flowers will not bloom blue. Also, when aluminium in the soil is used up the flower color will be red or pink again. If the soil is watered with aluminium salts aluminium is accumulated in the petals and the color turns blue. Only red and pink colored Hortensis are suitable for the "breeding" of blue flowers. They contain the dye component delphinidin.

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