Well...I'm going to be playing on and off again in a local open mic style blues jam a couple of times a month. Different players and songs at various points through a 3 hour night. I never know who I'll be playing with aside from the drummer and a guitarist. I'll be replacing the "house" bass player to give him a break.
He's got a very small Hartke combo on the floor (and the guitar player uses his combo as the stand for his little amp), so access to tone/volume controls is very limited. I may or may not end up replacing the current bass player on a more permanent basis as he plays with other bands and I don't gig at all right now.
Now I have a very nice Precision (P/J to be more precise), but there are two issues in using that. For one thing, due to some injuries and the lingering effects, sometimes reaching out more towards the nut can be painful to my fretting hand shoulder. Sometimes I have no issue, sometimes I do. It flares up randomly, never knowing when that might be. Secondly, the tone is rather baked in and works well with my amp and controls. I used it on one other limited fill-in gig with a different rig (no time to make amp adjustments) and the tone sucked in a major way.
I also have my 6 string, (active preamp) Ibanez GSR206 that is effectively an even shorter scale than any short scale bass because every note on my 4 string Precision can be found between the 5th and 12th frets on my 6 string. Just because I play a 6 string does not mean I'm playing solo stuff all over as I'm still a very groove oriented pocket player when not writing/recording. I can and will stay mainly between the 5th and 12th frets for the most part anyway. It also has a lot of extremely versatile tone control between the pickup volume controls, the tone control and the PHAT II preamp EQ control. With this I can more easily avoid messing with the Hartke controls.
Granted, I am in no way a blues purist snob, but some surely will be. For me it's more about being in the groove with the drummer and my 6 string will allow me to play without pain no matter what. Having the low B will also allow me to play any song in any key and keep the low end tight and moving along.
I'm pretty sure what the overall response will be on this topic (disregarding a few outliers), but I thought I'd throw it out there. I've been playing both of my basses along with recordings of most of the house band's stock blues tunes and both basses will work, but really only my 6 string guarantees me a nice deep low end groove and the ability to play anything regardless of how I'm feeling physically. I won't be playing all the tunes, all night and some different/new tunes may be played depending on what other singers, keyboard players and guitarists may be on stage with me at any given point.
I can't bring both as at the moment I only have one gig bag so I have to make a choice for the time being. Both, the drummer and guitarist are pretty open-minded as they host the jam.
So, for the heck of it, what's your opinion? 4 string P/J or the 6 string?
For you? Whichever bass feels most comfortable for you to play. IMO, nailing the "proper" bass Blues tone, while nice, is not really something that's vital for an open blues jam. Visuals? Well... a funky old, beat up P-Bass would be nice, but again? Open Blues jam... Whether you're comfortable with other people playing your bass, is for you to decide...
Personally? Very few of my 28 basses are not... suitable, tone-wise, for a Blues jam. That said? I'd probably pass on my "set-up like- it's- new" '78 P-Bass. The sound and visuals would be the cat's ass, but my 74 y/o back just can't deal with the weight for more than a song or two, any more. It would probably be a toss-up between 2 of my lined fretless basses. A) the 32" scale, '93 Canadian-made Fury Anthem, with tape wounds. It was built to sound like an upright bass - and Gawd, does it ever... Or, B) my '24 G&L Kiloton lined fretless. With La Bella LTF-4A's, it's awfully bluesy sounding - if I want it to be. As for other people playing my basses? IME, very few right handed players want to deal with a left handed bass, so...