Blackberry phones in the 8xxx and 9xxx series all support MP4 (MPEG4) and 3GP (H.263) video file formats without conversion. Streaming of these formats is also supported. BB support docs.
Although not impossible, in some circumstances it may be easier to find the porn you want while on a full-size desktop browser, and then transfer the clips to your Blackberry via USB or on a memory card.
NOTE: If you are not using the native Blackberry browser, some sites below may not display optimally on your device. Opera Mini, an alternative browser application, seems to identify itself as a non-mobile device and thus get served normal full-size website graphics.
Downloadable mobile porn is that which can be made portable and taken with you, like downloadable adult movies or short clips of porn movies.
Streaming porn phone services are those which require an internet connection, like streaming video or video on demand, and usually work best on devices with 3G or WiFi.