Textbox Nodes Won't Appear In Diagram

I am trying to create a simple venn diagram with the labels a,b,c with a U in the right hand corner of the box like this picture:

I have created the venn diagram the way I wanted to, but I can't create node labels that appear like in the image. Here is what I've made and the source code that I've been using. I've been playing around with different colors and I still can't see it.

\documentclass[12pt, tikz, border=5]{article} \usepackage{tikz} \begin{document} \tikzstyle{textbox}=[draw=black, fill=black, thick, rectangle] \begin{tikzpicture}[scale=4, blend group=screen] \fill[lightgray] ( 270:.7) circle (1) node [textbox] {\begin{minipage}{0.15\textwidth} $\boldmath{c}$\end{minipage}}; \fill[gray] (180:.5) circle (1); \fill[darkgray] (0:.5) circle (1); \draw[ultra thick, fill=black!100!] ([shift={(-0.1,-0.1)}]current bounding box.south west) rectangle ([shift={(0.1,0.1)}]current bounding box.north east) ; \end{tikzpicture} \end{document}