Here is a minimal working example on how to create such a block:

% Blocks without title in Beamer

% set a theme to have nice and different blocks


\begin{frame}{Blocks without title}
% create the main block
\begin{exampleblock}{Title of the block}
    This is the block with the main contents.

% create a other blocks, visually separated,
% that continues the contents of the previous block
    A natural but separated continuation of the previous block.

    This is a standard block without title.

    This is an alert block without title.
% since no title is specified, it will not appear the title band

Compiling this code yields:

blocks with empty title