So far, only a test for Sabino1 is available. There are tons of other known sabino types, but testing is not available.
Actually I have to say, that the OP's horse does not look like a Welsh cob but most probably either a Gypsy or Gypsy cross. Difficult to tell much from the pics, but he doesn't have the head of a Welshie and also has too much feather to be one. This colour is found commonly in Gypsies. Maybe he came from Wales, but I doubt he's a Welsh Cob. There are loads of Gypsy Horse breeders in Wales.
Another clue is that the seller called him 'Blagdon'. This is a term only ever used for Gypsy Horses. It denotes a horse of a plain colour (something other than tobiano) with usually, four white feet. I somehow doubt he's a true roan, although it is difficult to tell from the pics.
Here is an example of a blue blagdon. Just lighter in colour than our Myst. Lloyds | Vines Gypsy Horses
And here is his red blagdon daughter. Rococco
As you see, both have extensive sabino.