You do know that slavery was legal in Massachusetts until after the War of Aggression, don’t you? In fact, it was illegal to free your slave in Massachusetts if you didn’t post a bond to do so. Boston was 10% black at one time before the War of Aggression.
And are you aware that the 1790 federal census listed no slaves in Massachusetts? Technically you are correct - slavery had not been formally outlawed but it also wasn’t practiced.
Funny how even the first census in 1790 lists zero slaves in Massachusetts. Go here to page 6 of this PDF document if you don't you don't believe me.
You are partially correct that they didn't have a specific state law on their books outlawing slavery, but state court decisions beginning in 1780 based on interpretations of their 1778 state Constitution ended slavery in Mass.
So to the larger point, you are wrong. Slavery was not legal in Mass. The state courts ruled that it violated their Constitution.
Your Lost Cause BS propaganda has lied to you again. Why do enjoy being lied to?