blocking mode for bulk transfers possible? about lufa

[abcminiuser]

That's really up to the host driver implementation, but generally speaking, no. Bulk transfers are enqueued on the host side in advance and scheduled/sent via the low level USB controller driver via DMA as bus bandwidth permits. That's how bulk transfers end up having larger thoughput than other fixed interval endpoint types at the expense of higher latency.

The best way to approach this would be to add in an application level command/response layer above the CDC layer, so you can send (potentially multiple) commands with a given sequence number and have the device reply back with an acknowledges for each command that has been processed.