How to make qed box inside alignment environment

Question

My proof that I need to write is very short that it only consists of one alignment. Therefore, I want to put the qed box inside the alignment indented to the rightmost.

If I simply put \hfill\rule{2mm}{2mm} inside alignment environment, it does not indent to the end of the line.

\begin{align*}
    &= \textrm{a math equation here}\\
    &= \textrm{aligned properly}\\
    &= \textrm{blah blah blah}\hfill\rule{2mm}{2mm}
\end{align*}

I also tried to vertically space it using setspace package, however, it only seems to move any lines below it and not the square:

\begin{align*}
    &= \textrm{a math equation here}\\
    &= \textrm{aligned properly}\\
    &= \textrm{blah blah blah}
\end{align*}
\vspace{-12px}
\hfill\rule{2mm}{2mm}\\
More text here to compare blah blah blah blah blah blah blah blah blah blah

Also, it would be nice if I get put the Proof. on the first line of the alignment. Thanks in advance.

MWE:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{setspace}
\begin{document}
\textit{Proof}.
\begin{align}
    &= \textrm{a math equation here}\
    &= \textrm{aligned properly}\
    &= \textrm{blah blah blah}
\end{align}
\vspace{-12px}
\hfill\rule{2mm}{2mm}\
More text here to compare blah blah blah blah blah blah blah blah blah blah
\end{document}