If by 'blanks', you mean spaces, then you can test for a string containing only spaces by doing the following:
Replace all the spaces in the original string with nothing (i.e. a real blank), then check the length of the resulting string. If the length of the resulting string is more than zero, then there are characters other than blanks in the original string. If the length of the resulting string is zero, then it means all the characters in the string were spaces.
You will need the [replace text with text in text] block to achieve this.
In the first text box, type a space.
In the second text box, type nothing at all - leave it blank!
In the third box, place the text that you are testing
In the attached image, you can see that I have set the text attribute called "testblanks" to "some text". In the next line, I replace all the spaces in testblanks with a blank character, and I return the length. All you have to do is test the length!
I will reiterate that in the green code block, the box after the word 'replace' has a space in it. The box after the word 'with' has nothing in it - it is completely blank.