The second reason is heat. Black cloth warms up faster than light cloth in full sun, especially when it is dense and heavy. For a summer reception outdoors, lighter fabrics and lighter colours work better; we cover them in our guide to choosing a suit for a summer wedding.
The third reason is practical: black is unforgiving about fit. Sleeves that are too long, rippling across the back and trouser legs that are too wide all show at once. Navy forgives more, because it has depth and works better with shadow. If you are buying at the last minute and will not have time for alterations, black is a risk. We offer fitting and alterations on site in all seven Albione salons, but leave yourself enough time for them.
Consider your colouring as well. Very fair skin and light hair next to black and a bright white shirt create a strong, hard contrast. The shirt and tie can soften it, but if you are unsure, navy is the simpler choice.