Blackstrap Molasses

I've done that, too, and have actually liked the taste (I was making molasses cookies), but I prefer not terribly sweet desserts also. That said, you could probably dilute it to your taste with corn syrup or even mild honey. (I would do that rather than add sugar so you don't mess up the balance of sugar and liquid in the recipe.)

Assuming you're speaking about a baking/sweet recipe (mentioned sugar), blackstrap is not generally a good substitute for dark molasses; it's very bitter and intense. But, that in mind, bitterness is not necessarily a bad thing. It can be balanced out with sugar (brown or white). If it's a recipe like gingerbread (or something similar), the bitterness might actually be nice, but I don't typically like overly sweet desserts; it's a matter of preference.
Short answer: yes, in a pinch, you could compensate with added sugar. Depends on recipe, though. Too much added sugar could throw off the "baking formula"/