It is aluminum that makes blue hydraneas blue :-) While it is more readily available in acidic soils, sometimes it just needs to be supplemented regardless of soil pH. I'd try some aluminum sulfate (aka "hydrangea blueing") to see if that helps. You may not see much of an improvement this season but if you apply now, perhaps and if applied again in fall or very early spring, for sure for next season.

FWIW, 'Nikko' never produces a very intense or vibrant blue - it's pretty midrange. I'd describe as more of a sky blue than a gentian or caerulean blue :-)

Thanks, I'll try it. The blooms coming up now are very cloudy sky blue.