It's nothing to do with blends. Just create numerous rectangles of different colours, Drag select them and try changing their colour using the eyedropper Only 2 change colours & only one adopts the eyedropper selection colour, the second one to change only takes on a shade of that colour.
By the way, if you select the small purple square and copy it, then drag select the rectangles and Ctrl+Shift+A they all change colour as expected.
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