\begin{tikzpicture}
\usetikzlibrary{plotmarks}
\draw[color=gray, thin] (-3,0) -- (3,0);
%http://tex.stackexchange.com/questions/64567/how-to-draw-circle-square-and-triangle-marks-in-tikz-picture

\draw[color=blue!50, ultra thick] (-1.4625,0) -- (1.4625,0);
%\node[mark size=2.5pt,color=blue!50, fill=none] at (-1.5,0) {\pgfuseplotmark{*}};
%\node[mark size=2.5pt,color=blue!50] at (1.5,0) {\pgfuseplotmark{*}};

\draw[fill=blue!50,color=blue!50, ultra thick] (-1.5, 0) circle (.075);
\draw[fill=blue!50,color=blue!50, ultra thick] (1.5, 0) circle (.075);
\node[color=blue!50,] at (-1.5,0.35) {\textbf{a}};
\node[color=blue!50] at (1.5,0.35) {\textbf{b}};
\end{tikzpicture}