Guess what? it isn't.
At first glance and first feel, the paper appears to be the same as that used in their notebooks but in fact it responds totally different to fountain pen inks. I decided to do a quick test and got interesting results.
On the positive side, this paper (despite being off-white) really brings out the colour of the ink. The colours are a joy to look at. Also the paper has a pleasantly "grainy" writing feel, a sort of texture that offers lots of control while still being pleasantly smooth. No fibers, no plasticy feel as with some coated papers, just a really nice writing experience. The degree of control also makes it easy to jot readable notes when on the move (cars, buses, trains, planes).
On the negative side...
As you can see, there's a profound "mottling effect" that's caused by the ink being absorbed into the paper in some spots, which leads to colour differences. The inks that mottle most also cause the most bleedthrough and are pretty much unusable. The Herbin inks fall in this category, which surprised me a bit because I consider Herbin inks to be extremely well-behaved. But not on this paper...
The best-behaved inks on this paper, i.e. the ones with only very little mottling or no mottling at all, are Sailor Kiwa-Guro (bottom line in above image) and Platinum Blue-Black. Both are pigment inks. By far the wettest writing sample is from an '80s Sheaffer Targa M with Platinum Blue-Black:
There's a ton of Platinum ink on the page, but zero mottling, zero showthrough, zero bleedthrough. Remarkable. The only non-pigment ink that comes close is Pelikan 4001 Blue-Black, which also doesn't mottle or absorb. Diamine Ancient Copper is also well-behaved on this 'difficult' paper.
The purpose of this comparison is not to burn down Paperblanks diary paper, but to show how much inks differ from each other in how they interact with paper. Their properties really vary widely. The amount of ink deposited on the page (wetness) is *not* the deciding factor, it's really all about the chemical composition of the ink and how the paper responds to that.
Edited by TheDutchGuy, 21 December 2018 - 09:27.