You were using the arguments of textblock in the wrong way (please refer to the textpos package documentation for details). Anyway, your problem is that, according to your definition of varblock, this new block expects a title; for the title of your poster, obviously you don't need this field, so instead of using varblock you can simply use a beamercolorbox with the appropriate color (in this case, the color block body used to typeset the body of ordinary blocks). The code below illustrate this idea; of course, feel free to make the necessary adjustments according to your needs (I wasn't sure about the exact desired position for the blocks):
\documentclass[final, 12pt]{beamer}
\usepackage[size=custom,width=77,height=107,scale=1.4,orientation=portrait]{beamerposter}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{wrapfig,xcolor}
\usepackage{array,booktabs,tabularx,amsthm,multirow,amsmath}
\usepackage[absolute,overlay]{textpos}
\usepackage{mdframed}
\usepackage{tikz}
\usepackage{pifont}
\usepackage{multicol}
\usecolortheme{rose}
\newenvironment<>{varblock}[2][\textwidth]{
\setlength{\textwidth}{#1}
\begin{actionenv}#3
\def\insertblocktitle{#2}
\par
\usebeamertemplate{block begin}}
{\par
\usebeamertemplate{block end}
\end{actionenv}}
\begin{document}
\begin{frame}
\begin{textblock}{}(0.2,1)
\setlength{\textwidth}{75cm}%
\begin{beamercolorbox}{block body}
\vspace{-0.65cm}
\begin{center}
{\Large\textbf{\textcolor{violet!90}{Anticipated Complexities of Landfill to Estimate Fugitive $\mathrm{CH_{4}}$ using Micro-Meteorological Method.}}\\
\vspace{0.5cm}
\normalsize {\textbf{ M$^1$, T$^1$, J$^1$, R$^2$}}\\
\textcolor {black} {1.Uof A 2.Agri-Food Canada}}
\end{center}
\end{beamercolorbox}
\end{textblock}
\begin{textblock}{}(0.28,2.2)
\begin{varblock}[35cm]{\textbf{1. Introduction}}
blah blah
\end{varblock}
\end{textblock}
\end{frame}
\end{document}
To achieve the goal, you have to understand how the block colors are defined for the Boadilla theme.
Boadilla uses rose as color theme in charge of customizing the blocks; it sets:
\setbeamercolor{block title}{use=structure,fg=structure.fg,bg=structure.fg!20!bg}
\setbeamercolor{block body}{parent=normal text,use=block title,bg=block title.bg!50!bg}
As you can see, the background color of the block body depends on the background color of the block title. So, basically, you want to reproduce in the block title the same color of the block body:
\setbeamercolor{block title}{fg=darkred,bg=structure.fg!20!bg!50!bg}
where: structure.fg!20!bg is the original background block title color and !50!bg is added to let him be exactly the color of the background block body. Now, the latter should be:
\setbeamercolor{block body}{use=block title,bg=block title.bg}
and that's it.
A complete example:
\documentclass[10pt,xcolor=dvipsnames,pdflatex]{beamer}
\usetheme{Boadilla}
\usecolortheme{beaver}
\definecolor{darkred}{rgb}{0.8,0,0}
\setbeamercolor{structure}{fg=darkred}
\setbeamercolor{block title}{fg=darkred,bg=structure.fg!20!bg!50!bg}
\setbeamercolor{block body}{use=block title,bg=block title.bg}
\begin{document}
\begin{frame}{My pink slide}
\begin{block}{Oh my god}\centering
this is so girlish.\\
Especially when\\
the block is\\
so big\\
that it\\
is close\\
to the\\
red/grey\\
frame title.
\end{block}
\vfill
\begin{block}{}
Nice, bright block!\\
I like this and would not change its bg color.
\end{block}
\end{frame}
\end{document}
The result:
Best Answer
Change your definition of
varblockto