Pass option 'blockbg' through to colortheme package.

Also, use beamer-specific DeclareOption and error if incorrect options
are passed.

beamercolorthememetropolis.sty

@@ -9,12 +9,22 @@
 
 \ProvidesPackage{beamercolorthememetropolis}
 
-\newif\if@blockbg
-\@blockbgfalse
-\DeclareOption{blockbg}{\@blockbgtrue}
 
-\ProcessOptions\relax
+%}}}
+%{{{ --- Options ----------------------
 
+\newif\if@beamer@metropolis@blockbg
+\@beamer@metropolis@blockbgfalse
+\DeclareOptionBeamer{blockbg}{\@beamer@metropolis@blockbgtrue}
+
+\DeclareOptionBeamer*{%
+  \PackageWarning{beamercolorthememetropolis}{Unknown option `\CurrentOption'}%
+}
+
+\ProcessOptionsBeamer
+
+%}}}
+%{{{ --- Colors ---------------------
 
 % http://paletton.com/#uid=7050t0kkJkJsntwoyp6gYgoddc4
 
@@ -47,7 +57,7 @@
 \setbeamercolor{footnote mark}{fg=.}
 \setbeamercolor{page number in head/foot}{fg=mDarkTeal}
 
-\if@blockbg
+\if@beamer@metropolis@blockbg
 
 \setbeamercolor{block title}{parent=palette primary,bg=palette primary.bg!80!fg}
 \setbeamercolor{block title alerted}{use=alerted text,parent=palette primary,fg=alerted text.fg}

beamerthemem.sty

@@ -9,6 +9,26 @@
 
 \ProvidesPackage{beamerthemem}
 
+%{{{ --- Options ----------------------
+
+\newif\if@useTitleProgressBar
+\@useTitleProgressBarfalse
+\DeclareOptionBeamer{useTitleProgressBar}{\@useTitleProgressBartrue}
+
+\newif\if@protectFrameTitle
+\@protectFrameTitlefalse
+\DeclareOptionBeamer{protectframetitle}{\@protectFrameTitletrue}
+
+\DeclareOptionBeamer{blockbg}{\PassOptionsToPackage{blockbg}{beamercolorthememetropolis}}
+
+\DeclareOptionBeamer*{%
+  \PackageWarning{beamerthemem}{Unknown option `\CurrentOption'}%
+}
+
+\ProcessOptionsBeamer
+
+
+%}}}
 %{{{ --- Packages ---------------------
 
 \usepackage[no-math]{fontspec}
@@ -22,18 +42,6 @@
 \usecolortheme{metropolis}
 \usefonttheme{metropolis}
 
-%}}}
-%{{{ --- Options ----------------------
-
-\newif\if@useTitleProgressBar
-\@useTitleProgressBarfalse
-\DeclareOption{titleprogressbar}{\@useTitleProgressBartrue}
-
-\newif\if@protectFrameTitle
-\@protectFrameTitlefalse
-\DeclareOption{protectframetitle}{\@protectFrameTitletrue}
-
-\ProcessOptions\relax
 
 %}}}
 %{{{ --- Titlepage --------------------