This way you could set the desired color globally in your preamble, also valid for tables:
\makeatletter
\AtBeginDocument{\color{darkblue}\global\let\default@color\current@color}
\makeatother
If you just mean tables in addition, you could redefine the table environment to get all lines and text such as captions in the desired color, such as:
\documentclass{article}
\usepackage{fontspec}
\usepackage{color}
\definecolor{darkblue}{cmyk}{1.00, 0.50, 0.00, 0.40}
\let\originaltable\table
\let\endoriginaltable\endtable
\renewenvironment{table}[1][ht]{%
\originaltable[#1]
\color{darkblue}}%
{\endoriginaltable}
\begin{document}
\color{darkblue}
text
\begin{table}
\centering
\begin{tabular}{|c|}
\hline
1 \\\hline
2 \\\hline
\end{tabular}
\caption{Test table}
\end{table}
\end{document}
So you need to do it just once in your preamble, and if you decide to change the color later you can adjust it at this single place. Also, you could include \centering in this redefinition if you center all tables.