Hi there,

You do language override https://stackideas.com/docs/easyblog/administrators/how-tos/how-to-create-language-string-override or edit them at this path language/en-GB/en-GB.com_easyblog.ini on strings listed below.

COM_EASYBLOG_MAIL_TEMPLATE_NEW_POST_SUBHEADING="There is a new blog post published on the site"
COM_EASYBLOG_MAIL_TEMPLATE_NEW_POST_HEADING="New blog post posted"

Please give it a try and see how it goes.