I created a team called 'Blue' and assigned 8 silverfishes to the team (also tried with 1) and did both /testfor @e[team=Blue] and /say @e[team=Blue] and got no comparator output from the /testfor, and "That player cannot be found" for /say command.

All the commands I used:

1) /scoreboard teams add Blue
2) /scoreboard teams join Blue @e[type=Silverfish]
3) /testfor @e[team=Blue]
4) /say @e[team=Blue]