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][1] 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

 [1]: http://%20lists.gnu.org/archive/html/emacs-orgmode/2015-08/msg00768.ht%E2%80%8C%E2%80%8Bml