Syntax
set aString = aBlob.convertToString();
SemanticsReturns the blob as base64 string.
SubstitutablesaStringCan be any variable or object attribute having the type String. 
 aBlobCan be any variable or object attribute having the type Blob.
Examples
set aString = aBlob.convertToString();

Example File (Builder project E2E Action Language/BaseTypes):

<your example path>\E2E Action Language\BaseTypes\uml\converter.xml