Are you sending the email through an interaction? I'm setting up something similar and the title variable does not show up when sending through interaction but will show up if I send a test from send preview.

ET Support said I need to declare the title variable in the subject text box within the interaction. Do anyone know how to do this?

If you are sending your email through an interaction, declaring the variable in the body of the email will not produce the value in the subject line.

You must declare variable within the subject line text box.

%%[
SET @title =Field(Row(BuildRowsetFromXML(HTTPGet("https://www.yourblog.com/blog.xml"),"//item/title",1),1),"Value")
]%%

%%=v(@title)=%% %%=Format(Now(1), "MMMM d, yyyy")=%%