The answer that @Lgen gave works as far as the export is concerned, but it breaks visibility cycling (the headline level is lifted). As the thread on the org-mode mailing list suggests, empty headlines are considered evil. So I'd propose to insert something that has no effects export-wise, for example @@latex:@@. With this, you can also keep the tag, which I find useful as a marker of blocks in beamer.

** @@latex:@@ :B_block: :PROPERTIES: :BEAMER_env: block :END: Text in block without title 

The answer that @Lgen gave works as far as the export is concerned, but it breaks visibility cycling (the headline level is lifted). As the thread on the org-mode mailing list suggests, empty headlines are considered evil. So I'd propose to insert something that has no effects export-wise, for example @@latex:@@. With this, you can also keep the tag, which I find useful as a visual marker of blocks in beamer.

** @@latex:@@ :B_block: :PROPERTIES: :BEAMER_env: block :END: Text in block without title