In EUDP we generally uses UML and SysML diagrams.
Tutorials and Software
For Tutorials Please use your favourite search engine to search for "Tutorial UML" or "Tutorial SysML".
Over at Tutorialspoint you can find excellent tutorials in general and also for UML.
SysML is a bit harder to locate on-line resources suitable for self study.
Eclipse hosts Papyrus, which is a SysML and UML drawing tool of high quality. There is a guide in the project space on Eclipse.org , which can be used for getting started with SysML and UML Diagrams. Downloads are located in the same project space.
Block diagrams can be made in any tekst editor or drawing program like Libre Office Writer or Draw.
Dia can also be used.