Over on our forums user qrp has released a modified ExtIO that allows the direct sampling mode to work correctly in SDRUno. SDRUno is SDRplay's official software for their RSP line of software defined radios, but SDRUno can also work with ExtIO input dlls which allow other SDRs like the RTL-SDR to be used.
The commonly found RTL-SDR ExtIO however doesn't seem to work properly with direct sampling mode in SDRUno, so HF on RTL-SDR Blog V3 or other direct sampling modified RTL-SDR dongles is inaccessible. The new ExtIO fixes the direct sampling problem, and also enables a Remove DC algorithm to remove that center spike, which isn't an option in SDRUno.
To use the ExtIO simply extract the ExtIO_RTLSDR_u8.dll and rtlsdr.dll files from the zip file into a folder on your PC. Then from the Start Menu find the SDRUno (EXTIO) shortcut and run it. When it asks you, select the ExtIO_RTLSDR_u8.dll file. Note that you will probably need to use the older v1.22 SDRUno version as V1.31 doesn't appear to have an ExtIO version.