Font colour change caused by cite{}

Building on the (now deleted) answer of Maeve: the class sets the color to Black at the end of internal and external links (such as citations). This macro is defined in pdfcolor.tex as pdfsetcolor{cmykBlack} and cmykBlack is {0 0 0 1}, i.e., in the CMYK color model, with 100% Key and no colors. This however is not 'real black' or 'rich black', see for example xcolor black isn't black enough or Loading the color package changes the default color of the fonts. There are differences for printing and on-screen, and between viewers, but you could try different definitions of black to see which is best. Or cheat and use RGB (the PoS class does not mandate the color model in loading the color package). In this case changing the definition can be done easily by redefining cmykBlack (for CMYK) or redefining Black (for RGB).

MWE:

documentclass{PoS}

title{MWE}
ShortTitle{MWE}
author{speaker{MWE}}
abstract{MWE.}
FullConference{MWE}

begin{document}

%defcmykBlack{0.75 0.68 0.67 0.9}
defcmykBlack{1 1 1 1}
%definecolor{pitchblack}{rgb}{0,0,0}
%defBlack{color{pitchblack}}
section{MWEs}
    MWE cite{MWE} MWE 

begin{thebibliography}{99}
    bibitem{MWE}
end{thebibliography}

end{document}

Result:

For reference, this is the CMYK 1 1 1 1 version on an rgb 0 0 0 background:

But note that, if the article is actually printed on paper, the printer will not like that much ink, which may bleed on the page. But that is probably not really your concern as an author.