IBM 3270 Manual page 34

Using Keys with Black and Blue Markings
Most of the keys on your keyboard have black
markings. Some keys have blue markings, and
others have both. For example:
Print
.
-\
The keys with the black markings work as
labeled when you are using a personal computer,
a host computer, or a note pad session, o! when
controlling your work station in control mode.
In general, the keys with the blue markings work
as labeled when you are working with a PC
session.
The keys with both black and blue markings
work in two ways:
1. The blue markings work only in a PC session.
2. The black markings work everywhere else.
Using Keyboard/Control Program 3-5