I am working on automating my Somfy Blinds. The idea is to avoid using the remote control which works on 433.42 Mhz and use a simple RF transmitter and an Arduino. The thing is that I can not find any transceiver at such frequency. I only can get RF 433.92 Mhz transceiver like RFM69 @ 433 MHz among others that only mention 433 Mhz without precision. I read from other projects that this will work just fine for short distances, let's say 5-10m. So before buying a module that won't work I just wanted to drop the question here for the expert
A 500 KHz offset frequency would be substantially weakened by the filter of a decent OOK receiver at these frequencies, however, some lower cost appliances may use fairly wide receivers.
But that is really beside the point here, as the RFM69 is not a fixed frequency radio, rather it is configurable to a range of frequencies and modulation types, including the ~433.4 MHz or therabouts OOK used by a Somfy shade. So the premise of the question comes only from misunderstanding, and is moot.
Still, your task is not that simple either, as Somfy shades use a rolling code scheme. You will not be able to control one without transmitting a code which changes at each transmission in accordance with that scheme, and pairing your code sequence to the shade receiver.
Somfy use 443,42Mhz for carrier and RTS rolling code. Independantly of rolling code, 443 generic transmitter and receiver (as low cost chinese modules) use 443,92Mhz Frequency. Using this frequency on receiver as Somfy motor tuned on 443,42 works but only at short distance (around 5m depended of course of walls in sight of views) About RTS and rolling code you can read lot of info on https://www.domoticz.com/forum/
Somfy in the US have a product called URTSI II which can be intefaced to using RS-232, RS-485 or IR so that might work for you. Not sure where you are located but I believe Somfy US use 433.42 and here in Australia and possibly Europe use 433.92.
The README of the following github project contains some valuable information regarding this topic: https://github.com/Nickduino/Pi-Somfy In summary: Some cheap 433MHz transmitter modules are availabe. However those often use the frequency 433.92 MHz and require soldering skills to replace a 3-Pin 433.92 MHz chip with a 433.42 MHz chip using the same pin out. I was yet unable to find a source which sells 433.42 MHz transmitter modules ready to use which don't require such a soldering patchwork.