It should not show blog as tagline. If you look at the preview site, each of the portfolio pages have actually a custom page title. Whatever page title you provide when creating a portfolio page is what is shown on the top. Are you sure you chose the page template as Portfolio 2 Column or Portfolio 3 Column and not the Blog one when you created the page? You can see the example here –
https://www.livemeshthemes.com/portfolio-2-columns-full-width/
and all other examples of portfolio pages behave in a similar way.