Bilberry by Diamine is in the twilight zone between blue and purple. Far more dark than the European blueberry it’s named after. Billberry is a very nice dark ink. I’d call it a dark purple leaning towards blue, but it’s really a matter of perception. In any case it’s an elegant ink that works both for personal correspondence as well as note taking in the office. At first glance many will consider it a blue.
Like most if not all Diamine inks, this ink is very well behaved. No feathering, some shading, and minimal show-through. The ink is less wet than most, writes smoothly and has a drying time that's below average for Diamine (Clairefontaine paper). The ink is very afraid of water, even a little drop or some wetness will smear it and ruin your writing. Even days after writing.
Diamine’s Lilac Night was my first thought for a comparison, but it’s much lighter and more blue. Noodler’s Tchaikovsky (a not very vibrant purple) and iroshizuku’s Murasaki don’t come close either.
Bilberry is a beautiful, elegant color. Highly recommended!