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?
I’ve gone over the original records in Massachusetts many times. My ancestors were here in the mid-1620’s, and yes, some of them owned slaves.
Reading the old records you will find ample evidence for slaves in the households - even in Lexington the Cradle of Liberty. When it became necessary they became called indentured servants. The slaves stayed with the families because they couldn’t earn a living and the towns didn’t want them. BTW - it was illegal for colonists to buy anything from an Indian, a black or a mulatto. Too much ‘went missing’ in the households.