Given that “blindly agreeing” is generally used to refer to agreeing to someone else’s ID without being able to confirm it for oneself or even checking whether it seems plausible, what would “blindly disagreeing” look like?
Disagreeing requires making a decision about what ID to enter, because (by definition) it is different than existing IDs. You cannot just click a “thumbs down” button on iNat – you have to actively suggest something else. So in most cases there is some minimum of thought that goes into entering a disagreeing ID. (Note that this is not about the rightness of the ID or the reasoning behind it: an ID may be entered with thought and intention but still be incorrect or based on incomplete, bad, or outdated information; this does not make it “blind”.)
In addition, in general people tend to find it psychologically more difficult to disagree than to agree, because disagreement is conflictual (calling someone else wrong/potentially making them unhappy/causing loss of face/etc.) while agreeing is experienced as positive and affirming. Many of us are socialized that it is impolite to refuse a request or correct someone. So disagreeing means actively going against our own inhibitions and will therefore typically be an intentional, considered action rather than something entered thoughtlessly.
Occasionally I do see users enter a CV suggestion that conflicts with previous IDs provided by someone knowledgeable. From my experience, I would call this an uncritical (“blind”) use of the CV but not necessarily a “blind disagree” because the intent does not seem to be to disagree, but merely to add an ID. Generally these IDs seem to be entered by newish users who do not fully understand IDs and how they are calculated, much less what a disagreement is. Based on discussions I have had with users, in at least some of these cases they are not aware of the previous IDs that they are disagreeing with and they may be trying out the computer suggestions without realizing that they get added to the observation record.
As far as whether people disagreeing should include a justification for doing so, this has been discussed thoroughly on multiple occasions in the past, along with why people often don’t – in particular the fact that it often feels like a waste of time when experience teaches us that many users don’t care or won’t see it.
For what it’s worth, if a user has explained their ID and I disagree with it, I personally do try to make sure to add a note about why I don’t think it is that species – because the user has indicated that they are interested in actually understanding the ID, not just putting a label on the observation regardless of its correctness. And because the note is in a sense an invitation for further dialogue, and I would feel like I was being rude if I ignored that.
Some IDers have workflows that mean they may not see comments. Not my style and I think it is problematic, because they might be overlooking other important information (like notes about what the organism of interest is), but it does mean that the lack of response to your comments may not be intentional.