Content blocks can be shown via an IF ELSE
statement, but the trick is to store the block in a variable, and then render the variable.
%%[
var @myBlock, @myVariable
IF @myVariable == "TRUE" THEN
SET @myBlock = ContentBlockByID("1234")
ELSE
SET @myBlock = ContentBlockByID("5678")
ENDIF
]%%
%%=v(@myBlock)=%%