Second cousins would be the " _third degree of blood relationship_ ". People who were related more closely than that were not permitted to marry (without special dispensation).
!Consanguinity
* Source - Wikipedia
As the above table shows, second cousins were not the only family relationship that would qualify as _third degree of blood relationship_ , but in cases like this it probably means _Second cousins_ , or perhaps _Second cousins Once removed_.
The ages of the individuals (if provided) might give a better idea about the exact relationship.