Only Krema I is intact, but it was heavily modified in 1944, to be used as an air raid shelter. It was clearly painted and had plaster repairs, since it was converted back to show the Leichenkeller.

The Majdanek delousing chamber/gas chamber, has large areas of wall with no staining.
How does anyone actually know that Krema I did not look similar, in 1943? Does anyone even know how long staining takes to appear?
Only a small part of Krema II can be accessed, and has no staining;

But, that cannot rule out other parts had staining. When the other Kremas are ruins, it again cannot be ruled out that they never showed any staining at all. It may have been faint, as it takes time to appear, but who knows, when the building now looks like this;
For all any of you lot know, there were signs of the walls of the Krema gas chambers turning blue.