Yo fellers, molasses is beet sugar. Blackstrap is fed to farm animals to get the Mg, cal, K, and iron benefits. Store brand molasses is usually not blackstrap, I've heard about it being available, and I'v even seen the briar rabbit brand Bud recommends with blackstrap, but I've never actually seen one for sale.
The thing about molasses is that it acts as a chelator - it helps the plant nutrient molecules in the soil/feed better. Similar in action to humates. In a healthy organic setup, there exists a thriving 'micro-life' population. The micro life processes the raw organic nutes (guano, blood meal, kelp) into chemical/molecular forms that the plant can use (soil-food web). The plant uses energy to produce sugars/starches at the root zone to feed the 'micro-life'. The main benefit of molasses, besides the Potassium, is to provide sugars to the rootzone that the plant would otherwise have to spend energy to produce. The plant does not eat the sugar, or absorb the sweetness or anything.
I use the 'briar rabbit full flavored brand', and I did the math one time. I used 21g as the weight of a tablespoon of BRFF, and calculated a nute profile of 0-0-1.4 per tablespoon, with I THINK 1% iron and .5%cal and Mg (the iron, calcium, and magnesium had to be figgured turning "% US RDA" into weights via the usda w.s) I believe that blackstrap is ~double the cal and mag. BRFF is double the nute value of grandma's robust, which is almost double grandma's in the orange label. I have used all 3 with no noticable difference, but the BRFF is available to me for 30cents less a bottle, and I like the taste better in oatmeal and in weed cookies and stuff. I am convinced that the nute values are negligible if you have the K covered in your grow, but the chelation/sugars help a lot. I have seen 'organic' molasses that was ~0-0-7, less cal and mg, whatever, but it was over 6 bux a bottle, almost 3times what I pay for the BRFF, and I have plenty of K