I play in multiple jams weekly, which I do because it is such a challenge.
It's important to have a decent idea of the various standard chord changes that are particular to the styles of music you like. For the blues it's pretty easy, as they're typically I, IV, V chords, or their alternates, which repeat in various ways. Throwing in 7th and 9th chords colors them. Rock will have a larger (or smaller amount of) variety, depending on the songs. Country and jazz can be even wider ranges of chords. The important issue is you have to be able to do those patterns without thinking, so you can listen to what's going on around you and think ahead to what you want to play when it's your turn to step out.
"I'm concerned that learning scales like the back of my hand may be detrimental to originality."
You do know that the majority of good players do know scales, and the licks we all play incorporate sections of those scales, right? People might not know the names of the scales, but that's only of secondary importance, because being able to play the notes is the important thing. Being able to play the common scales like major, relative minor, dorian, lydian, mixolydian, plus pentatonic and "blues", will carry you a lot farther than not knowing any of them and originality is better expressed by knowing your scales and knowing when to apply them, than by having absolutely no clue and fumbling around.
Jam at home with your favorite songs and learn the basic melody so you can play it over the solo space. Then start expanding on it by playing embellishments and harmonies. This isn't so much so you can play that same melody in a jam, it's so you can learn where your fingers need to go when you hear a pattern or melody in your head as you're playing in the jam.
Practice alternate fingerings for scales in the style too. When a song is going fast and furious, you need to be able to grab the notes wherever you hear them, not just because your fingers are in a certain place on the neck. That will help your comfort level and let you concentrate on getting the melody out of your head.
Don't try to be a star when you're up there. My attitude is it's all about the song, making it come alive and taking it somewhere as a group. It's not about my ability to solo, it's about my ability to fit into the song and help it breathe. I lock onto the drummer and bass to form a good back line behind the singer, and when I'm signaled then I say my bit, then STFU and slide back into rhythm. The singer should be calling the solos and breaks so pay attention and be ready.
Perhaps the hardest thing to learn is to relax and be comfortable. The jams I go to welcome people of all experience levels. I hadn't played in about 25 years when I started again, so I was really rusty and was very self-conscious. The leaders of the jams I go to remember what it was like and put together sets that work for everyone. In the beginning I was nervous to the point of feeling sick. Now I'm one of their regular work-horses; On some nights they call me up set after set to help others who are new or inexperienced. And, we're not just playing blues now; Two weeks ago we did an extended version of Billy Cobham's Stratus, which I didn't catch the name when they called it, and then recognized the riff, told myself I could do this, and ten minutes later we were laughing after having had so much fun.
Part of being comfortable is knowing your guitar and how to get a variety of sounds from it quickly. I have a Strat I love, so it's my go-to guitar, and if I want something darker and nastier sounding I'll take a T3 semi-hollow body. Strats are endemic but too many people play only on the middle pickup, so know the sound of the other settings if that's what you play.
Amps. Some jams don't let you bring your own. I don't play at those, because the fingers, guitar and amp make the sound. I've had jam leaders turn the amp down so low it was inaudible on stage, and after having that happen multiple times before I'd even played a note, I found other places to go. Have a rig you can drop on stage in a minute and have powered up and be ready to go in three. Be tuned, and very quietly noodle around and listen to the song suggestions. In short, be ready to go.
So, hang in there, find a place that soothes your heart, relax, learn to transfer your melodies to your fingers, be humble and listen to the others on the stage, then put your heart into it.
I'll be standing to the side during jams and have non-musicians ask how the heck do we do it, do we practice together, do we learn all the songs, etc. I laugh because to me it's so simple, we just listen and make it about the song and the rest just happens. Invariably though, they are amazed we can figure it out on the fly and walk off shaking their heads.