Are simple macros sometimes incompatible with TikZ?

I am making a whole bunch of graphs whose formatting I would like to control from the preamble.

But this seemingly trivial endeavor fails in the code.

What's happening? Why does the first `TikZ` picture work while the second fails?

```
\documentclass{article}

\usepackage{tikz}

% THIS IS THE MACRO THAT FAILS
\newcommand{\BlueLine}{line width = 2.8pt, blue, opacity=0.35}

\begin{document}

% THIS COMPILES
\begin{tikzpicture}
\draw[line width = 2.8pt, blue, opacity=0.35] (0.3,-3)--(7,3);
\end{tikzpicture}

% THIS DOES NOT COMPILE  
\begin{tikzpicture}
\draw[\BlueLine] (0.3,-3)--(7,3);
\end{tikzpicture}

\end{document}

```