Black Guillemot AKA "Tystie"

To be honest, until I saw these swimming & diving in Lerwick Harbour I never knew they existed,
Tystie is the Shetland Dialect name for the Black Guillemot
So, are not all Guillemots Black, and the Black Guillemot has white patches on it's wing?
Other sightings today, The usual suspects of Fulmars & Gannets an odd Skua going around looking for something to beat up and some Puffins whirring across the wave tops like demented clockwork toys.
It was good to get off the boat for a wander round Lerwick for an hour or 2. But we are away back out to the worksite North West of the Shetlands.