I am using a Mac on Mojave 10.14.6, it seems that blhost does not run reliably on Mac... When running blhost commands from SPT, frequently it will error out.
I am using the bootloader/flashloader on USB, and tracing the USB bus, I can see that when blhost fails, there is no USB packet sent.
I try running the blhost command manually from Terminal, and I can see it fail in the same way, only about every 10 executions will blhost send the USB packet, and then get a successful response from the flashloader.
I tried the same from a Windows PC, and blhost runs correctly every single time using the same setup.
I see there is a bug in blhost for Mac that someone identified here: https://community.nxp.com/message/1272499
I tried to make these changes and recompile blhost from the mcu-boot tools included with SDK 2.7, but now get a segfault when using this new compiled version.