Legend Lore would reveal information about the legendary creature and is not prevented by Mind Blank due to Legend Lore being cast on the caster, not the legendary creature
I found this answer to a similar question: Why does Mind Blank stop the Feeblemind spell?. It that question it states that Mind Blank prevents all effects from Feeblemind due to the fact that Mind Blank
foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.
Using that statement as a basis for what Mind Blank prevents, it would seem that Legend Lore would not be prevented by Mind Blank. Legend Lore states that you can
Name or describe a person, place, or object. The spell brings to your mind a brief summary of the significant lore about the thing you named. The lore might consist of current tales, forgotten stories, or even secret lore that has never been widely known. If the thing you named isn't of legendary importance, you gain no information. The more information you already have about the thing, the more precise and detailed the information you receive is.
By definition, for the Legend Lore spell to take effect, the creature must be legendary, which you said it was in your question. This lore, as written in the description of Legend Lore, does not have to come from the creature itself, therefore never affecting the mind blanked creatures mind, and not being prevented.
Legend Lore also never states that the information is taken from the creature being targeted by the spell, which would mean it is not prevented by Mind Blank. Mind Blank can prevent a wish spell if it tries to affect the creatures mind, however Legend lore does not state it affects the legendary creatures mind.
Nepene Nep makes a good point in his answer saying that
Legend Lore is a divination spell, and Mind Blank blocks divination spells
However, legend lore has a range of self, therefore meaning it is cast on the caster, not the legendary creature.
Additionally, I would say if the creature truly is legendary, then preventing Legend Lore from working by using Mind Blank would make things a lot harder on the PCs to find information about it. The lore they get might not be very specific, but can include current tales, or forgotten stories about it, made up by other creatures that this legendary creature might have no knowledge about.
For example: You could have a very friendly, yet shy, legendary Yeti, but the local stories about him all say that he is a stone cold killer.
The players would most likely learn that information about him being dangerous, rather than friendly.
Another point that has been raised is that Mind Blank also mentions it prevents
spells or effects of similar power used to affect the target's mind or to gain information about the target
I have assumed that Mind Blank would only prevent information from being extracted from the legendary creature, rather than about the legendary creature. I could not find any resources, or similar question about spells used to gain information about a creature, if that creature is not the target of the spell.
My argument is that Mind Blank would only prevent information from being gained from the legendary creature, and that Legend Lore would provide information that is known by other creatures. If no other creature knows any information about the legendary creature, it would by definition not be legendary.