LPM agar is generally recommended for isolation of listeria species. If I had gallbladder samples, I would pulverize and homogenize the sample, then streak on to LPM agar for isolation. For the blood samples on blood agar, I would probably do the same thing, but you will need to be adept at spotting the morphology of the species you are looking for on the blood agar. You will most likely end up with multiple species on your streak plates. For this reason, I would also recommend using LPM agar for the blood samples.