You didn't tell us what software you were using to try and decode the APRS signals.... ?
The Rigblaster M8 is a soundcard interface..... ie. something to handle the hardware interface between your radio and the soundcard built into your computer. But this alone wouldn't be enough to decode APRS or packet.....because you need some software working in between to act as a "soundcard modem". There are a handful of programs which will potentially do this. WB8LBZ's comment below references MultiPSK.....which sounds like it would potentially do this.(ie. decode simple packet). However, all you would be seeing (if operating properly), is the text rolling down on the screen..... it's not the same as operating APRS software.
There are people who are using "soundcard-based" modems for packet/APRS but this takes a little more effort and there are some additional layers of software needed. I agree with one of the other comments made that it would be easier to use a proper hardware based TNC instead.
However, maybe it would help if you could tell us what you hope to achieve? Are you looking simply to decode the packets? Are you wanting to interface things with some specific APRS software? Were you looking to try and wire this into a vehicle/mobile station for tracking purposes?
73, Tino VE6MB
Calgary, AB