Expressions available in Email Body

Update: I found this - Workflow expression

I know I can use ‘If’ expressions in the body of an email workflow. https://help.appsheet.com/en/articles/2697069-template-if-expressions

Can I use ISBLANK? or other expressions?

I am doing it in the Email Body not in a template.

Yes

The email body is a template, if I recall correctly.