Disable field if other is filled

You can simplify the logic you are using for a direct assignment, based on the comparison of the number of characters written in codigo . You can find out the number of characters by accessing length of value of field:

codigo_interno.disabled = codigo.value.length > 0;

So you have disabled in green if the size is greater than 0 and false otherwise. You can even avoid possible inadvertent spaces at the beginning and end by calling using the trim function to cut them:

codigo_interno.disabled = codigo.value.trim().length > 0;

I also advise you to use the onkeyup event to make it more interactive and change as you fill in, and register with Javascript to organize it better.

Example:

var codigo = document.getElementById("codigo");
var codigo_interno = document.getElementById("codigo_interno");

codigo.addEventListener('keyup', function() {//registo  é feito com addEventListener
  codigo_interno.disabled = codigo.value.trim().length > 0;
});

<label>Código</label>
<input type="text" id="codigo">

<label>Código Interno</label>
<input type="text" id="codigo_interno">

Note that I put the html fields search with getElementById out of the function because this is only done once.

Edit :

If you need the fields to be used alternatively, that is, typing in one will disable the other and vice versa, just apply a function similar to the codigo_interno field. This function will disable the codigo field if you have jobs written in codigo_interno :

var codigo = document.getElementById("codigo");
var codigo_interno = document.getElementById("codigo_interno");

codigo.addEventListener('keyup', function() {
  codigo_interno.disabled = codigo.value.trim().length > 0;
});

//agora uma função para keyup muito semelhante para o codigo_interno
codigo_interno.addEventListener('keyup', function() {
  codigo.disabled = codigo_interno.value.trim().length > 0;
});

<label>Código</label>
<input type="text" id="codigo">

<label>Código Interno</label>
<input type="text" id="codigo_interno">

In your code it was already set to input internal_code to be disabled, so it would not change the status because you wanted to switch to a state that was already set.

document.getElementById("codigo").onblur = function() {libera()};
function libera() {
  var codigo = document.getElementById("codigo");
  var codigo_interno = document.getElementById("codigo_interno");

  if (codigo !== "") {
    codigo_interno.disabled = true;
    codigo_interno.value = "produto...";
  }
}

<label for="codigo">Código
<input type="text" id="codigo">
</label>
<label for="codigo_interno">Código Interno
<input type="text" id="codigo_interno">
</label>

Just modify:

if(codigo != ""){
    codigo_interno.disabled = false;
}

by

if(codigo != ""){
    codigo_interno.disable = false;
}

Property is wrong, just typing error