The ornamental design for an electronicblackboard, as shown and described.
1. Electronic blackboard
1.1: Perspective
1.2: Rear perspective view
1.3: Front
1.4: Back
1.5: Left
1.6: Right
1.7: Top
1.8: Bottom
1.9: Alternate configuration
1.10: Another alternate configuration
The broken lines indicate portions of the electronic blackboard that form no part of the claimed design; reproductions 1.9and 1.10show the display part of the design in an alternate position.