I'm an IT Consultant seeing something very similar with one of my clients:
Running 6.0U2 + patches (build 4510822), Horizon 7.0.2, and a wide array of clients (various Wyse terminals, laptops, thick clients, desktops)
Most of our client types connect fine, but older Wyse devices specifically get black screens - but only in Multimonitor mode and only when using a 4.x client. If using a 3.x client the screens work fine in any mode.
Very bizzare. I have not pushed the November ESXi update stack to the hosts yet, but I am curious if there are any patches in there that would resolve this issue.