nice. so why does all recipes specify not blackstrap?
Blackstrap molasses has a stronger flavor than regular molasses. You could try using less (I'd start with 1/2 the amount of molasses called for) and then adjust your seasonings as needed. Otherwise, this recipe uses blackstrap molasses. I have not made the recipe myself so I can't attest to the authenticity of it or how it will taste. https://www.goldenbarrel.com/recipe/molasses-baked-beans/
Can you get maple syrup? or some other type of syrup? Sorghum?
Blackstrap molasses is too strong for the flavor of Boston style Baked Beans. I would use it half & half with a milder syrup.
I think using 1/2 blackstrap and 1/2 honey or rice syrup would work. I was born and raised in New England and completely understand the love for boston baked beans!!
Can you find sugar beet syrup? In Germany it is called Goldsaft and is shelved with the sugar and flour in grocery stores. I use this as a substitute for molasses in making ginger snaps and other recipes. See http://www.grafschafter.de/grafschafter-goldsaft. I think you could also use treacle--Lyle's is the brand I see most often.