You can define text styles—including the font, point size, color, and formatting–for any kind of text in your Blurb book. Click in the text box and select Customize Styles from the dropdown menu on your BookSmart toolbar. Enter your desired text style options for Heading 1 & 2, Body, Header/Footer, and Caption and click OK. Now, any text that is designated as Heading 1 & 2, Body, Header/Footer, or Caption will pick up those attributes. See the screenshot example below.
To apply these text styles universally across your book, you should mark each text component as Heading 1 & 2, Body, Header/Footer, or Caption. Passages that don't pick up your chosen font/color/size are probably not marked appropriately.