Some nurseries rate Nikko Blue hardy to zone 6 while others rate it hardy to zone 5. From the posts that you see here, it seems that the ones who advertise Zone 5 are stretching things a little. Some individuals are successful in Z5 but there are many complaining about lack of blooms in there.

If you really want to try, experiment to see if the following three suggestions will help; then follow the fourth one if you still get no blooms.
1. make sure you do not prune them on or after August-September since that is when new buds are developed
2. fertilize once, in June, with manure or cottonseed meal but that is it. You can utilize weak fertilizers (liquid seaweed, liquid fish or coffee grounds) but add no fertilizers at all after August-September. This allows the plant to go dormant and be prepared for winter. If the plant is caught in growth mode because of too much fertilizers when cold weather comes, the temperatures may kill the buds.
3. micro-climates: consider planting close to the foundation so the heat from the house will help it through the winters; consider other places where it may be protected on two sides by walls, structures, etc.
4. if the above suggestions do not help, prepare to follow winter protection techniques (see the link below) or switch to varieties like All Summer Beauty Series, Endless Summer Series, Penny Mac, Dooley, etc (these bloom on new wood).

Luis

Here is a link that might be useful: Winter protection techniques

As luis indicated, Nikkos are not reliable bloomers in zone 5. I grow a couple, but I have reliable bloomers, too, so I never have a bloom-free summer.
Even if you winter protect if there's warm spring weather then a frost, say buh-bye to your blooms.
All Summer Beauty is my fave. Stays small and is covered with blooms every year no matter what.

Hi Foxglove -
I'm amazed at how your Mom's Nikko Hs reliably bloomed all those years in Zone5 ... I hope you'll correct me & say she lives in a milder Winter, part of the country! My 18y/o Nikko bloomed only 4x & gave me just few huge/semi-huge clusters in her 1st 16 years. I learned about 'over-wintering', from this forum just recently & for the first time had blooms galore last season!

Do take them, if she'll just pitch them. :-) Definitely, you'll need to Winter protect!!!
Keeping serious mind, all of Luis' point by point recommendations, you might want also to plant them together if not in a row, to make life a bit easier for you, to climate-control their site.

A gentleman named Phil, in ME, constructed cosy cages for his collection, using PVC tubings & surrounded them with some protective insulating sheets. He claimed that the neat looking protective Winter homes, for his treasures, were easy to disassemble & store for the season. I urged him to post the photos he Emailed to me, in Hayseedman's 'Overwintering' thread, so others with rows of beautiful treasures, in foundation beds to protect, could see. BTW 18y/o Nikkos are huge with, surely, huge root balls to dig up with care ... help will be nice to have!

Good luck & Congratulation to you, if you succeed in adopting them!!! <:->