I've noticed defects on many a professional bladesmith blades and, when communication was possible without possible misunderstanding, I told them politely and respectfully.
Up to now, it's been always appreciated.
Many times a defect had gone by unnoticed in the frenzy that always precedes a knife show, and the knife was instantly removed from the table with thanks.
Actually, in a couple of cases, it's been the maker himself that has asked me my opinion on other knives afterwards.
In my experience, a good craftsman will always appreciate respectful, constructive criticism of his work and will keep anybody with observation and critic capacity in higher esteem than those who praise their work independently from its quality.