RE: Trouble with XPS 15 9550, USB-C/Thunderbolt 3, and external monitors
I have the same problem with a DisplayPort connection using the XPS 15 9550 and an external display. Are you running Windows 10? I've speculated that the half completed Control Panel ==> Settings overhaul isn't fully blocking the "closing the lid" power-management functionality, even with the "Do Nothing" setting in vogue. The laptop may be erroneously and ineffectually sending the signal to the display to sleep. But if the same problem exists in Windows 8.x, then that theory seems unlikely. It's a pity Dell doesn't offer the option in BIOS to select what the laptop does when the lid is closed.
I have the TB15 Dell Thunderbolt Dock too. Whether I connect the DisplayPort cable to a Thunderbolt 3 adapter or the Dell Thunderbolt Dock the results are the same. I don't think the Dock plays a role. You say the blinking off and on problem doesn't happen with HDMI, that would indicate that the Thunderbolt 3/USB-C port, or more likely its controller software (which Dell updated only a few days ago), is at fault. I suspect Dell will get this ironed out in time. They are really good about updating their drivers, firmware etc. But they need to know about the problem.
I contacted support and had a very long dialogue with them that lasted over a month. But because my machine is 4K and my external monitor is 4K, I never thought to use the onboard laptop's HDMI 1.4 port, which does not support 4K at 60Hz. But I'm going to try it tomorrow. By the way, it's definitely not your monitor. I've experienced the same problem with three different Dell 4K monitors now.
I'd appreciate it if you came back and posted what version of Windows you're using.