Hi,
The To: field of the e-mail message requires a full e-mail address and you will need to take the value from the person column, and using the "AD Users" SmartObject, retrieve the e-mail address for this user from AD.
I'm not entirely sure if the AD Users object is available in K2 blackpoint, but I believe it will be. Look through the available SmartObjects using the Environment Library in K2 Studio to determine if it is available. There should be a method which can retrieve the user's properties and you would set the input property to the account name of the user and the return property as the email property of the user.
Give this a try and let me know if it works.
Regards,