It is never positive during the primary stage when a chancre is present. It is always strongly positive during the secondary stage, and will remain strongly positive for years after. In someone who has been adequately treated, the blood test will slowly diminish in reactivity, but will often never go away entirely, so it is common for us to find a positive test in an older person who will tell us that he or she had syphilis many years ago, and got treated. The standard screening blood test occasionally produces false positives in people with other conditions. Therefore, it should always be confirmed with a second blood test called a treponemal test.
Many labs do that test automatically on everyone who is positive on the screening test.
I assume that you had such a confirming test done. A person with syphilis is only infectious to others during the primary and secondary stages, when the chancre and the rash are teeming with spirochetes. After the rash clears, our body's immune system tends to hold the organism in check, and the infected person no longer sheds spirochetes, and therefore cannot infect someone else. If you were diagnosed by blood test, and don't have a rash, I would not be worried about infecting your partner. Nonetheless, he should be checked by a blood test, since perhaps it was he who infected you at the beginning of your relationship.