Using a mouse click somewhere dead on the page, such as in between interactive elements, will achieve the removal of focus (blur) from whatever element had it..."
Well, kind of.
As you know, Tab just moves focus through the focusable elements in the document, stepping through each focusable child of each HTML element.
However, the browser still keeps track of the region of the document that's last selected, even if it's not focusable—this means that at all times, some region is the last selected (at page load, this can be thought of as the document itself).
You can see this behavior yourself: if you click in a web document just before the footer links, then press Tab, the footer links gain focus instead of just continuing from wherever focus was last left.
This behavior illustrates that clicking a "dead" region of the page is not so much "blurring" the form fields, as much as it's selecting a new element of the document which may or may not be focusable.
So what about your question?
I have tried every reasonable (to me) combination of keys while a form input has focus, but other than unintentionally doing all kinds of other things, I've found no way to simply stop focusing on the input.
To consult an authoritative source here, the W3C states the following about this exact situation, in section 7.3.2, Focus Management:
When an element that is focused stops being a focusable element, or stops being focused without another element being explicitly focused in its stead, the user agent should synchronously run the focusing steps for the body element, if there is one; if there is not, then the user agent should synchronously run the unfocusing steps for the affected element only.
If I'm reading this right (and I may not be), it seems that the W3C would suggest that the document's
<body> tag should be focused after the blurring of some element in the document. In that case, the
tabindex would reset, and restart at the top of the document. As a user, I'd think it strange to have a shortcut that blurs where I am and sends me back to the top of the
tabindex. I'm guessing that browsers chose to just do nothing instead of doing something else.
Perhaps you can glean additional information from this source that may make things clearer, but this was the best I could find on the matter.