High RBLs are indicative of reagent contamination. While it may not be from the instrument itself as the noting of a thorough cleaning. I know from past experience that the chemicals used to make the nitrate color reagent (or any reagent) can become contaminated or expire. Especially with the sulfanilamide used to make the nitrate color reagent. I would inquire if any of the chemicals used were purchased from a different manufacturer than previously used and to ensure the CAS numbers are correct (defined within our methods). Also this color reagent is extremely light and temperature sensitive so it must be made and stored in amber glass and refrigerated. Otherwise it will turn a pink color causing high RBLs. I also suggest wrapping in foil the smartchem reagent bottle used to contain the nitrate color reagent.
Another possibility is the quality of the DI water used. The same DI water batch must be used for all reagents, standards, and blanks.
The average RBL value is deducted from all sample results.