Are black windowboxes unsuitable for a west facing windowsill?

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I bought black windowboxes because It thought they looked smart but 2 summers in a row the plants have failed despite copious watering and having water crystals in the soil. The plants were lobelia, trailing verbenas and dianthus. I adore trailing verbenas particularly. I'd love to have huge overflowing windowboxes to cheer me up whilst I do the washing up. It is a very hot location though with the afternoon sun. Are the black boxes attracting too much heat?

They're plastic

They will be making the soil much hotter.
You could try lining them with sheets of expanded polystyrene to give some insulation (white packing material).

Plants generally like cool roots to thrive.