The smoothest writing, blackest ballpoint ink is the Schmidt EasyFlow 9000 and it fits any Parker compatible pen. I have many but have been using the Rotring 600 recently and love the grip, weight and quality. Amazon has them for $24 today, howeve price changes everyday so use CamelCamelCamel to get one at the price you want.
No matter how smooth hybrid inks are they do not compare to a gel and IMO Pentel Energel makes the best gel pens and refills. I even had pens made $30 each for Pentel or Pilot G2 refills. I love the blue and violet.
As far as a fountain pen blue ink - Diamine Oxford Blue is my favorite.
The smoothest and blackest writing hybrid ink refill is the Schmidt Easyflow 9000 and it fits into all Parker compatible pens and there are a lot out there. If you want a gel then I suggest Pentel Energel. If you want disposable pens you can get the Pentel Energels as cheap as $1.33 each. TheMonami FX 153 are $2 each and accept the Schmidt EasyFlow 9000.
Want a better pen for Parker refills then get a Rotring 600 (prices change everyday/hour and you should be able to get one for ~$25) - too much then here is a $7 alternative and for a Pentel you can get the business version (metal) for $9
Rotring 600 or 800 (prices move around on these two pens on Amazon daily so suggest you use CamelCamelCamel depending on whether you wan to spend $50 or $30 and whether you want a twist or click pen). Both take Parker compatible refills and I use either the hybrid ink Schmidt EasyFlow 9000 or the Monteverde Capless Ceramic Gel refill.
If you want a custom pen, I have bought from these Etsy sellers Sonoran Pen, Xezo, Soots (made to take Pentel refills) - the rest take Parker compatible refills. Picture 1 Picture 2 Picture 3 Picture 4
If want to change pens, I would change refills also. IMO the Schmidt EasyFlow 9000 writes smoother and blacker than the Jetstream and it is cheaper if you use the Jetstream refill for Parker compatible pens.
Suggestion: Rotring 600 - price runs all over the place on Amazon but you should be able to get one for <$25 if you use CamelCamelCamel - this is my go to everyday pen.
A different option is the uni Alpha Gel with a Pentel Energel refill - this is a favorite among many.
You could always take a look at Smootherpro - they have pens that take Parker G2 and Pentel Energel/Pilot G2 refills
