So if I'm reading this right, you have unflavoured 50/50 pg/vg nicotine juice @ 36mg/ml and you have blueberry flavouring concentrate that you wish to add to the unflavoured juice to get a final product that is both very strongly flavoured *and* still at 36mg/ml strength.
If I'm wrong, please forgive me, however if this is the case.....
If you add ... let's say 10% flavouring by volume, your final juice will be 32.4mg/ml strength as you are diluting it. At 20% it will be 28.8mg/ml.
If you want to stay up around 36mg/ml while having a large amount of flavouring you'll need to start out with a higher strength unflavoured VG nicotine liquid, add your flavouring and then dilute with PG to the required volume.
Here's how I would make 100ml of your juice.
I'd buy a bottle of 48mg VG nic and a bottle of 100mg VG nic and some plain PG from a nice vendor such as RTS Vapes.
I'd mix the two different strength nic juices together at equal volume (25ml each) so that I'd now have 50ml unflavoured nic juice @ 74mg/ml
Then I'd add however much flavouring is required eg: 20ml for 20% flavouring and then I'd add 30ml of plain PG.
The final product will be approx 100ml of 50/50 pg/vg juice at 37mg/ml assuming the flavouring liquid is PG based.
If after this you decide that 25% flavouring is what you desire, adding another 5ml of flavouring to the final product brings the total strength down to a fraction over 35mg/ml.
While testing I'd probably use smaller quantities so if you divide all quantities by 5, you'll end up with 20ml of juice at the same strength.
Of course, if 25% flavouring is actually what you need, then it is simpler to purchase a bottle of PG based nicotine as well as a bottle of VG based nicotine liquid twice as large, both @ 48mg/ml.
Simply mix half VG nic, a quarter PG nic and a quarter flavouring and you have 36mg/ml flavoured juice 50/50 with 25% flavouring.
Hope this is what you needed and I'm not just telling you what you already know
Cheers
Let us know how you go