The blockform selector is just 20em high, and it centers vertically. If you have a lot of regions (like omega), that looks broken. I changed
line 171 in context_reaction_block.css like this

#context-blockform  .context-blockform-selector {
  height:20em;
  height:100%; /* pike hack */
  overflow:auto;
  }

and that looks much better

*-pike