The blood refers to his death. Alternate translation: "because the lamb had shed his blood and died for them" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word "testimony" can be expressed with the verb "testify." Also who they testified about can be stated clearly. Alternate translation: "by what they said when they testified to others about Jesus" (See: [[rc://en/ta/man/translate/figs-abstractnouns]] and [[rc://en/ta/man/translate/figs-explicit]])
The believers told the truth about Jesus, even though they knew that their enemies might try to kill them because of it. Alternate translation: "but they kept testifying even though they knew that they might die for it"