The ornamental design for a blackbox for vehicles, as shown and described.
1. Black box for vehicles
1.1 : Perspective
1.2 : Rear perspective
1.3 : Front
1.4 : Back
1.5 : Left
1.6 : Right
1.7 : Top
1.8 : Bottom
1.9 : Reference
Reproduction 1.9 is a reference view showing the subject design installed on the overhead console for vehicles. The broken lines depict unclaimed environmental subject matter.