Larry, I'm routing for you here, but I'm pretty black & white on this. You've committed to this gig, you need to honor that. No subs, no excuses. Illness or some other unforseen event is a different thing, but breaking a personal "contract" on the basis of something better coming along is a huge no no for me. Rightly or wrongly, you've already had loyalty expectation issues with your rock band guitarist, so this would be pulling his string big time. In my opinion, he/they'd be right to be pissed, I would be. In taking this other gig above one you've already contracted to do, you're essentially saying to your rock band, "guys, I'm here with you until something better comes along". How would you expect them to react to that? I know I'd be looking for someone else next day. There is a wider long term issue here too Larry, your credibility as a reliable player who conducts himself honorably within the business.
Now, on the other side of things, what you do with your time when not contracted to work with the rock band is entirely your affair & non of their business. If they're threatened by that, tough crap, the decision is taken for you, & you've walked away with dignity & reputation intact. So, the question is, how valuable is this rock band to you?
My gut feeling is you're a blues player through & through. Not only do you clearly excell in the genre, it's also your passion & source of greatest pleasure. I'd go to the blues band leader and explain the position with your rock band, tell him you'd like to play in his band for every gig, tell him if that was on the table that you'd consider leaving your rock band to allow that to happen, but would need the security of knowing you had the gig as a permanent band member. If he values you as a player, you're essentially asking him for a reciprocal commitment. Use that leverage respectfully, & if you get that deal, then leave the rock band.
Choices choices, I know, but sometimes you need to take one step back to take two steps forward. You know your motivations Larry, so go with what floats your boat, but you can't have your cake & eat it!