I have some used reed switches with no datasheets. all of them have a black mark on one side. I want to know what does this mark mean?
They don't. Have a broader look at reeds from other sources. I have a bunch of them here, and install them professionally for burglar alarms. They either have no markings, or sometimes the manufacturer's details. They don't have little black dots. Can I suggest Google Images "reed switch"? You'll see loads of examples, all without black dots. A reed switch is symmetric, non directional or un polarised. They work either way round so don't require one end to be marked.
As to a getter. No. Getters are only for maintaining vacuum. A reed switch contains an inert gas (typically nitrogen). The wiki entry mentions high voltage reed switches that operate in a vacuum, but I don't think that this is one of those.
It could be a testing mark, perhaps maybe. Could also be a spider.
A normal reed switch should not be orientation dependant.
The paint dot is most likely a manufacturer model code designation.
However a SPDT reed switch has one non (or less) magnetic terminal at the two wire end (the NC terminal).
A normally closed reed switch often is a SPDT type with the NO terminal cut short. In this type the two end wires may have different magnetic characteristics and a dot might provide some strange value. The pictured device looks like a normal NO switch.
While the picture does not show any difference it is possible that in a switch where one terminal is mobile and the other rigid a manufacturer might see fit to mark the relevant end, again I cannot see any use for this information.