My guess too much water

Yeah, sounds like over watering to me too. You should not need to water every day this early in the season, even in Mel's mix. Stick your finger in up to the first knuckle, if there is some moist soil sticking to your finger, you should be good on water... You may need to water a bit more often when the seeds are germinating because they don't have any roots to absorb the moisture from deeper in the soil, but once the plants have some good roots, you should not water as often. Mel's mix does dry out kind of quick, so in the heat of the summer you may need to water daily, and definetly use lots of mulch.

I agree with the too much water....might be damping off. Also, beets in general IMO are kind of a pain. I've grown them successfully, but have learned that I need to plant them very densly, and gradually thin them over a period of a few weeks. A lone beet seedling is very vulnerable. Also constant hilling of the row to keep them sturdy. I love them, but don't mess with beets much, or carrots either anymore. Maybe when I'm retired......just too much work for now.

Forgot to come back and say thanks for the replies. It's funny, I thought overwatering too, but then the day after I posted this, it started raining heavily and did so off and on for a few days. When the sun finally came out again, the beets had grown like crazy and the stems were thicker than before! Even my husband remarked on the monster beet plants (he's dying to get at the greens, but I want the beets!). So I have no idea. I did pull the ones that had the blackening stems--maybe I just culled the weak ones and the rest are stronger?

I didn't know that about hilling them--I thought that would rot the stem. I'll do that as they get bigger and seem to need help.

Thanks for the advice!