For seed starting, I would rather go with the miraclegro + sand or perlite if you already have the potting mix. Blackkow is basically N rich compost and doesn't have all nutes, micros needed for plants to grow.
From your description of the current mix, the yellowing seed leaves could very well be due to excessive nutrients. The seed leaves feed the seedling, but if there us excess of available soil nutrients, the seed leaves are superfluous and drop off (but in doing so, the seedling loses the seedleaves' chlorophyl/solar energy collectors.
I don't uppot to richer mix until 2nd pair of true leaves start to grow. In any case, you don't want to over fertilize the tomato and pepper seedlings because they grow too fast -- too tall to fit under the lights, too wide/compete for space and shade each other, and also too spindly and weak.
I mix 1 part miracle grow potting soil, 1 part black kow, 1 part peat moss..
IMHO I think this is good combination for older seedlings IF you also add 1 part sand/perlite. (Though, personally, I would use coir instead of peat moss. You could also leave out the peat moss/coir altogether.)