Does a man need to reveal his blemishes before marriage?
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Let's start by saying that every situation is different and, for any real shidduch, you will want to speak to a rav who knows the situation well.
This being said, R Yirmiyohu Kaganoff at yeshiva.co has a very nice summary of the issues. He frames it as follows:
Honesty (emet) requires one to be truthful in his dealings and one may not deceive someone about a shidduch by misinforming the other party - he notes however there might be allowances not to disclose everything to avoid creating disputes (machlokes), embarrassing someone or hurting a person’s feelings or reputation
Not misleading someone (geneivat daat) means not overselling one party in a shidduch or making false promises
Fraud (onaah) means one must notify the other party of information that might be of concern. He quotes Sefer Chassidim (507) that "do not hide medical issues from the other party to which they would object enough to decline the shidduch"
He also notes shidduchim are subject to some exceptions
In most instances, there is no requirement to notify the other party or a shadchan of any of these blemishes at the time a shidduch is suggested. Many contemporary poskim contend that one should reveal this information after the couple has met a few times - about the time the relationship is beginning to get serious.
To come back to your question therefore, one has to disclose significant hidden blemishes that would be of concern to the other party once the relation gets serious.
See the original for sources and more examples.
See also a similar compilation from R Chaim Jachter with more sources and examples of blemishes to be disclosed (e.g., significant diseases, reproductive issues)