SAC also has the internal functionality of a calculator. The "Blackboard" is simply a feature that saves user defined variables. These variables can be manipulated and used in calculations. Results can be fed into SAC commands for further data processing. Thus, SAC has built into it a simple programming language.
The syntax for manipulating blackboard variables is relatively simple but sometimes obscure. Refer to the SAC manual for details.
Variables can be defined, evaluated, and put into simple equations. There are also IF, ELSEIF, ELSE statements in addition to DO loops. Sometimes it is a real tradeoff whether to write a fancy SAC macro to utilize blackboard variables or just use your favorite programming language to do the same thing.
Examples are given in the Macros section of these webpages.