Blimey 2 out of 3 Sundays rostered! Are Sundays in or out of the working week? I'm guessing in?
Well... the answer is that they are outside, but there is a compromise over the commitment to work them.
It is a Thameslink roster from a couple of years ago. At Thameslink, Drivers who joined before 2016 must commit to work 8 Sundays per year, and Drivers who joined after that must commit to work 12 per year. The remaining ones are voluntary still.
This depot's answer has been to put as many Sundays in the roster to provide the biggest choice for which 8/12 you'd be commiting to.
If they were
all committed (as is the case in other TOCs who have 'committed Sundays') then I would say there would be less Spares (marked As Required [AR] in this roster) on Sundays, but it would certainly be a lot more Sundays worked than the Blackburn one (probably 1 in 3).
However, with Sunday frequencies being touted as being increased, committed Sundays across the country might end up looking more like this roster.
I know the original question was about Blackburn, but thought this was perhaps worth including given the thread title.