Every pure Forest Lord has the chance to develop May Cast Lifeblade as a talent, but not every Forest Lord pet actually will.
What you may need to do is hatch your pet with another Forest Lord. A hatched pet sometimes develops different talents than its parent pets. You have an especially good chance of getting it if one of the parents has already developed the talent you're looking for.
A final option is to hatch your Forest Lord with another life pet. For example, if you happen to have a Yuletide Spirit pet, she also has that talent and a hatch between the two might bring out that Life Blade talent.
Alia Misthaven