For 'cartridge only' pens, I use Private Reserve Velvet Black cartridges.
There may be blacker blacks than these, but both look deep and dark and magnificent when I set nib to paper, and they are black enough for me.
Interesting that you've found black ink more apt to degrade over time. Curious, I paged quickly through some of my older journals. I have to admit that Aurora Black ink on a 17-year-old page is paler than on a page in my current journal. But the blues and browns and reds all look paler as well. I'm okay with that.
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