# How can I get sharp corners inside and rounded corners outside a tcolorbox?

You can replace the frame code so that only the left border is drawn, which removes the artefacts and allows you to have the sharp internal corners:

\documentclass{article}

\usepackage[skins]{tcolorbox}

\newtcolorbox{blueleftbox}{%
enhanced,
boxrule=0pt,
leftrule=5pt,
sharp corners=west,
frame code={
\fill[blue]
([xshift=5pt]frame.north west) --
([xshift=5pt]frame.south west) --
([xshift=1mm]frame.south west) arc
([yshift=-1mm]frame.north west) arc
cycle;
}
}

\begin{document}

\begin{blueleftbox}
This is a tcolorbox!
\end{blueleftbox}

\end{document}