How long do you give your plants to climatise ?

I'd agree that for most plants adding some soil (e.g. John Innes) to they fluffy type of compost is a good idea as it provides structure and holds moisture better.
I always add some JI and perlite to fluffy compost for everything except seeds.

But with blueberries a very low pH compost is vital, otherwise they will not be able to absorb nutrients. The leaves will yellow and the plant will slowly weaken.
It's due to the very low compost pH that an ericaceous fertilizer is needed for them.

I had to repot my 3 plants during COVID so could only get the same stuff you have from Westland.
It has been fine.
It needs more frequent watering and I usually need to water daily in Summer.
When growing, it's better to overwater them than underwater them.

You can buy ericaceous John Innes that is soil-based but it's not widely stocked.