Which waterproof black ink can you use to refill brush pens is one of the most frequently asked questions I get.
Let me put out a disclaimer first before I recommend anything. The inks mentioned below are all pigmented ink. So there's still a chance that they may clog your brush pen, especially if you leave the ink in the pen for long periods of time without using the brush pen. I suggest trying the ink out on the cheaper Pentel Pocket Brush Pen before putting the ink in expensive ones. And take note of how often you use your brush pens.
The type of ink you want to avoid are those ink that leave crusty bits around the cap. India ink and sumi inks are strictly no-no. Fountain pen inks, which are usually dye inks, are safe to use in brush pens, but they are not waterproof and may have lightfast issues.
As for the pricing, they will vary depending on where you buy them of course. I've provided Amazon links where you can compare the prices.
This ink is dark, dries relatively fast, is matte when dry and flows well. When used to cover large areas, the application is also quite even.
My second recommendation is the De Atramentis Archive Ink which is made specially for fountain pen use.
This ink is a bit more patch compared to the Sailor KiwaGuro. But if you apply your strokes slower, you'll see less of the of patchy look.
Third recommendation is the Platinum Carbon Ink. Performance is quite similar to De Atramentis.
Noodlers Bulletproof Black flows really well but is too patchy. And the ink dries too slowly.
Another recommendation is the Rotring Ink that is made for the Rotring technical pens (eg Isograph). Performance is similar to Sailor KiwaGuro. You can get this ink in large 250ml and small 23ml bottles.