The 27-year-old has yet to sign a new deal and, with his contract running out in June, the centre has been linked to other clubs.
Somerset Live claim that Bath have made Joseph an offer but it is not as lucrative as his current deal, with the West Countrymen attempting to alter their wage structure as they look to make room for other high-quality additions.
However, Blackadder is hopeful that the experienced international, who is currently out of action after breaking his foot in April, will put pen-to-paper on a new contract and extend his stay beyond 2019.
“Things are looking quite positive,” he told Somerset Live. “I can’t really say much more than that at the moment because nothing’s done and dusted. We’re still a work in progress. We’d love to keep him.”