If the source of this question was something prepared by a teacher for a class (which is common here), then you should be aware that teachers don't always use completely formal terminology or notation when making handouts. They would like to, but if they have to prepare it themselves, there are practical difficulties.
All this is to say that the lack of morpheme-defining curly brackets { } around a formal morpheme name like {-ed} for past tense or {-en} for past participle doesn't mean that -en should be taken literally. It's a name, not a description. That's what the curly brackets signify.
But, en famille, once you've learned what { } means, if you're dealing with morphemes a lot, the brackets tend to be omitted, as long as they're understood. This is one of the normal gotchas in learning anything formalized -- when do you formalize and how much do you formalize? More important, when do you not? The answers are usually dictated by the group of people who use the terms frequently, in their pragmatics.
So, clearly the past participle of blow uses an /n/, not a /d/; it's not a regular allophone of the past participle {-en} (which has the same 3 forms as the past {-ed} morpheme), so it must be a different allomorph. In this case, the allomorph is simply /-n/; there's no /e/ that needs deleting. That "e" is simply part of the name of the past participle suffix morpheme, and not a phoneme to account for.