Does Morgana's Black Shield activate Windspeaker's Blessing?

Yes, Morgana's Black Shield is counted as a shield, and Windspeaker's Blessing specifically states that

Your heals and shields are 10% stronger. Additionally, heals and shields on allies increase your target's armor by 5 - 22 (based on level) and magic resistance by 2.5 - 11 (based on level) for 3 seconds.

So, as it is a shield, it will be modified by Windspeaker's Blessing.