It felt like a downbeat night at the SFJazz Center. Joni Mitchell, the guest of honor at the annual black-tie gala held on Friday, remained hospitalized, the state of her health still shrouded in mystery after she was found unconscious in her Los Angeles home more than a month earlier. As the center’s benefactors and a small assemblage of local celebrities mingled outside the building at Fell and Franklin streets, the mood on the red carpet — actually blue for this occasion — was noticeably more somber than previous years. The windows of the vacant building across the street, meanwhile, were decorated with pictures of Mitchell at various stages of her career along with #THANKYOUJONI. A small shrine of flowers and handmade greetings adorned the chain-link fence below. It was fitting, then, that inside the Robert N. Miner Auditorium, where the members of the SFJazz Collective and other performers had gathered to pay tribute to Mitchell, 71, the music was predominantly moody and meditative.