April Shower Bring May flowers, but we're not talking about that kind of bloom!
What is rubber bloom? Most nitrile (NBR), highly saturated nitrile (HSN/HNBR) and neoprene (CR) rubber materials undergo a process called "blooming" when they are stored. "Bloom" is a milky dusting of dry powder on the surface of the rubber. This is typically caused by unused vulcanizing agent(s) migrating to the surface of the rubber part.
Bloom is entirely superficial. Since blooming is normal and does not affect the function of a rubber seal, it is not considered a defect and is not considered a contaminant in the rubber material. If the look is not acceptbable, you can wash the part in water or light mineral oil to remove it.
Bloom can also be caused when a lubricant is added to the eleastomeric compound for internal lubrication. By design, the added friction-reducing agent will not be chemically compatible with the base elastomer. this conflict means that the agent will separate itself and "bloom" up the the O-ring's surface. Continual blooming of the agent keeps the seal's exterior coated with lubricant, making the O-ring slippery and less inclined to stick during startup.
Lubricating can be either organic or inorganic. Widely-used organic lubricants include amides, waxes, esters, powered PTFE and mineral oils. Inorganic agents include graphite and MoS (Molybdenum disulfide). The lubricant must be compatible with system fluids to avoid leaching of the agent and also compatible with all adjacent surfaces to avoid structural damage.
Year-round, "bloom" is a natural process in rubber storage with no effect on the rubber seal.