Throw it out along with the soil.

Thanks. Do you have any idea what it is? The soil is from a brand new bag of Promix.

And the seed was quite an expensive hybrid seed (Chocolate Sprinkles), so this is actually only one of two seedlings I grew.

I threw out the seedling, but now I wonder if I should not use the Promix anymore (of which I still have a huge amount left, since it was a new 3 cubic ft. bag). I just saw that two other seedlings (of a different variety) also show this symptom. If anyone know what this may be, and if it might have come from the soil, I would be very grateful.

Blistering on tomato leaves can just be from overwatering. The rest of your leaves look fine. I'd just ignore it, and clip off the blistered leaves if they bother you. Oops, maybe try to resurrect the plant you threw out?

Thanks so much. That actually makes sense, because we were just away on a 4-day trip, and before we left I had really soaked the pots heavily to ensure they wouldn't dry out while we were away. I haven't watered them since though (it's now been six days), as they were still moist when we came back.

Not sure if I can resurrect the seedling from the trash, but I still have another one in a small cell tray, which I'll transplant tomorrow. I only have room for one plant of this variety.

Thanks again, I hope you are right! :)