I recently bought a Pilot Petit2 Sign Pen in blue-black. You can pretty much talk me into to buying any pen if its blue-black (check!) and under $3 (check!). It is just a fraction of an inch longer than my Kaweco Sport so its nicely pocket-able.
It also fills that in-between width: the point is wider than a LePen or Sharpie Pen but much narrower than a Sharpie fine point marker. Am I the only person who thinks there’s a gap in their pen-nib-width options?
If blue-black is too austere for you, there are lots of other color options and the Petit2 line includes brush pens and fountain pens. All are refillable with ink cartridges though I’m not sure what the lifespan is on the fiber-tip.