After researching into how spell effects stack, I find some ambiguity regarding certain spells that have multiple possible effects.
Combining Game Effects (p. 252). This is a new subsection at the end of the “Combat” section:
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the ‘Combining Magical Effects’ section of chapter 10 in the Player’s Handbook.
There are, however, no references in deciding how to determine what "the most potent one" may be when not using raw numbers (such as with paladin auras).
This is also applicable to spells like contagion, which inflicts a "natural disease" (and you can be afflicted by multiple natural diseases).
If both effects of blindness/deafness cannot influence a character at the same time, how do you determine which one takes effect (assuming both casts are at the same spell level)?