Macros that contain catcode 1 and 2

Some TeX primitives (e.g., \vbox, \hbox) can be used with implicit braces: \bgroup and \egroup:

\documentclass[12pt]{book}
\usepackage{pgffor}

\newcommand{\blockStart}{%
  \vspace{15pt}%
  \vbox\bgroup
}

\newcommand{\blockStop}{%
  \egroup
}

\begin{document}

\foreach \n in {0,...,34}{
\par text line
}

\blockStart
Keep these lines together\\
Keep these lines together\\
Keep these lines together
\blockStop

\foreach \n in {0,...,33}{
  \par text line
}

\end{document}

This is also, how environment lrbox works (\hbox + \bgroup and \egroup). However, \bgroup and \egroup cannot replace curly braces (category code tokens 1 and 2) for macro arguments.

This works:

\documentclass[12pt,letterpaper]{book}
\usepackage{pgffor}

\newcommand{\blockStart}{\begin{samepage}}
\newcommand{\blockStop}{\end{samepage}}



\begin{document}

\foreach \n in {0,...,35}{
\par text line
}

\blockStart
Keep these lines together\\
Keep these lines together\\
Keep these lines together
\blockStop

\foreach \n in {0,...,33}{
\par text line
}


\end{document}