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.

 

I thought I read somewhere that Gardening Which had done some trials and found not much difference between ericaceous and ordinary compost, but I can't find it now.

 

There might not have been a great deal of difference when regular compost was peat-based (peat being slightly acidic) although I imagine the pH would vary depending on how much lime was added by the manufacturer. Peat-free composts are much more variable in many ways so I wouldn't be surprised if they were very variable in pH too. I would stick to ericaceous for any plants that need it to be acid (rather than just prefer acid), and preferably a soil-based one for long-term planting, ie longer than a season. Or add some bagged topsoil.