http://docs.blesta.com/display/user/Company+%3E+Emails#Company>Emails-EmailTemplates
That's why there's two tabs on the welcome email, if your email doesn't support Html it will show the text. (Both templates are sent in the email).
HTML The HTML version of the email.
Text The text version of the email.
You also see the text version on Blesta, if you go to the client profile and check the email history you see the text version.