Most people don’t know how potions work. I think it’d be safe to tell someone to add thestral blood without telling them what for, and if they’re someone as mundane as Lily Potter they would never figure it out.
Except that Lily did figure out what the substitution would do. She immediately figured out that it would make the subject sick and might be deadly, and she eventually figured out that it would make the change permanent (or whatever good thing it did) and would probably not be quite deadly.
Most people don’t know how potions work. I think it’d be safe to tell someone to add thestral blood without telling them what for, and if they’re someone as mundane as Lily Potter they would never figure it out.
Except that Lily did figure out what the substitution would do. She immediately figured out that it would make the subject sick and might be deadly, and she eventually figured out that it would make the change permanent (or whatever good thing it did) and would probably not be quite deadly.