In the Reference Guide for the R2400 (it is a PDF file), in the section “Changing the Black ink cartridges to suit the paper type”, it says:
“If an ink cartridge is temporarily removed from the printer, be sure to protect the ink supply port of the ink cartridge from dirt and dust; store it in the same environment as the printer. Store ink cartridge with its label facing upward. Do not store cartridge upside down. Note that there is a valve in the ink supply port, making covers or plugs unnecessary, but care is needed to prevent the ink from staining items that the cartridge touches. Do not touch the ink supply port or surrounding area”.
Somewhere the Epson literature (I cannot find where just now) it also suggests storing the cartridge in a plastic bag. I have been using a Ziplock type of bag for this purpose and (following what the statement above statement says) I have not covered the ink supply port. I usually forget to follow the advice about the label facing upward but so far (about 11 months) I have had no problems switching/storing black cartridges.