Sorry for reviving this thread, but I stumbled across it and had to jump in and defend our work a bit
But first – thank you so much for even showing enough interest to carefully look through the sample. I'm J. A. Valeur, founder of Eventyr Games and Bob's partner on DELVE. I designed the oozing kobold, so I'll give you the logic behind the design decisions.
1. In 5E stat blocks, whatever comes after the "Hit:" only happens on a hit, so if the kobold doesn't hit, it doesn't matter if it rolls a 1 or 19, the effect won't happen – just like you wouldn't have the target roll a Con save vs Poison damage if it isn't hit by the assassin's dagger.
2. I agree, the "rolled 15 or higher on the attack roll" is a bit ambiguous and could be phrased better. Generally, 5E will always refer to dice roll + bonuses as "the result", while "the XXX roll" refers to only the die roll itself. Often, however, 5E wording often includes a "on the dX" to minimize the ambiguity, which we could have done as well – and, based on the feedback here, probably should have (so thanks to your feedback, we'll have time to correct that in the final book!)
3. As for the not having a save part, that is intentional. It's a 1-turn blind that has a 30% (or 50% with advantage) of happening – it's the sort of situation where having to roll constant Con checks will be annoying, while this is easy for the GM and the players to remember. Blinded is not nothing, but most of the time it's just disadvantage on attack rolls (albeit for spellcasters, it can make it impossible to case specific spells) – and it's just one round.
All that to say, thank you for your feedback and your interest in DELVE! We've updated the sample to make things a bit clearer
Appreciate it!