Pilot Iroshizuku Tsuki-yo vs Diamine Registrar's Blue-Black

These solve different problems. Pilot Iroshizuku Tsuki-yo is the better daily ink: a saturated teal-green with smooth flow and gentle shading, but no water resistance. Diamine Registrar's Blue-Black is the better document ink: a true water-resistant iron gall mark for a few dollars, at the cost of regular pen flushing.

This comparison contains affiliate links. As an Amazon Associate and partner of other programs, NibPath may earn a commission at no cost to you.

At a glance

Spec	Pilot Iroshizuku Tsuki-yo Ink Check price on Amazon	Diamine Registrar's Blue-Black Ink Check price on Amazon
Price	$28-$32	$8-$12
Our rating	4.8 / 5	4.5 / 5
Best for	Daily journal writers who want a distinctive, smooth ink	Writers who need water-resistant or archival ink for documents
Ink type	Dye-based, smooth flow	Iron gall, oxidizes to a permanent mark
Water resistance	None; smears when wet	Genuine, once fully oxidized
Maintenance	Standard flushing only	Flush pen every one to two weeks to prevent corrosion
Street price	$28 to $32 (50ml)	$8 to $12

The two options in depth

4.8 Editorial score Amazon

Pilot Iroshizuku Tsuki-yo Ink

A saturated teal-green dye ink from Pilot's prestige Iroshizuku line with smooth, consistent flow, moderate shading, and zero shimmer. One of the most-recommended inks for daily writers who want a distinctive color without fuss.

Best for Daily journal writers who want a distinctive, smooth ink

Outstanding flow consistency across nib sizesBeautiful teal-green color with gentle shading in broad nibs50ml bottle lasts most writers many months Not water-resistant; smears when wet

$28-$32 Check price on Amazon

4.5 Editorial score Amazon

Diamine Registrar's Blue-Black Ink

A traditional iron gall blue-black ink that dries to a permanent, water-resistant mark. Widely used for legal documents and archival writing. Requires regular cleaning to prevent corrosion.

Best for Writers who need water-resistant or archival ink for documents

Genuine water resistance once fully oxidizedArchival quality, suitable for legal documentsVery affordable per milliliter Must flush pen every one to two weeks to prevent corrosion

$8-$12 Check price on Amazon

Which should you buy?

Buy the Iroshizuku Tsuki-yo if this is a journaling or correspondence ink and you want the smoothest possible writing experience in a distinctive color; the flow is the benchmark dye inks are measured against. Buy the Registrar's Blue-Black if you sign documents, fill out forms, or need an archival mark that survives a spilled coffee, and you are willing to flush the pen on schedule. Many writers keep both: the Pilot in a daily pen, the Diamine in a dedicated document pen.

Common questions

Can I leave Registrar's Blue-Black sitting in a pen?

No. Iron gall ink left to dry in a pen can corrode steel and gold nibs and clog the feed. Flush the pen every one to two weeks if you write daily, or empty it entirely before storing the pen. The Iroshizuku has no such constraint and tolerates sitting far longer.

Which ink is friendlier for a beginner?

The Iroshizuku Tsuki-yo. It flows reliably across every nib size, requires no special maintenance, and is one of the most forgiving inks to learn with. Iron gall inks like the Registrar's reward writers who already have a cleaning routine.

Is the Registrar's Blue-Black really water-resistant?

Yes, once it has fully oxidized on the page. The iron gall chemistry bonds to paper fibers as it dries, so a finished line resists water meaningfully. Fresh, undried ink will still smear, so give signatures a minute before exposing them to moisture.