The text at the top of your Tumblr Blog is the same as the title of your blog and is also the same text used for the browser tab text for your Tumblr blog. You can make changes to either the title or to the HTML to affect what the tab text is for your blog.
Your Tumblr blog title is set in the customization setting for your blog along with the description of the blog. Whatever text you have for the blog title will appear on your browser tab for that blog as well as in all the search results of the blog. If you want to change your blog title you can log in to your Tumblr account, access the settings, select your blog, choose to customize it and change the title to whatever you want. Note that changing the title does not affect your blog's URL.
At the same spot where you can change your title, you can also change your blog's description. The description won't show up on your Tumblr blog's tab on your Web browser, but it will be used in search results. It will also appear underneath your profile picture and above any separate pages you might have on your Tumblr blog.
Using the HTML editor on Tumblr you can customize the title for your blog by using images, special text or colors to highlight the title of your blog. If you use an image for the title of your blog the text won't display on your page and you can use the title to add custom text that will appear on the browser tab for your Tumblr blog. You can find out how to edit the HTML code to change your blog's title (link in Resources).
Custom Tab Text