Russian arms export organizations have never exported Su-27 fighters to Angola. That's why Unita rebels' claims about one Su-27 (callsing Blue-272) being downed with Russian pilot at the controls doesn't correspond to reality. This was reported to ITAR-TASS by "Rosoboronexport" official.
Angola Air Force has a variety of jets in its possession including: 20 - MiG-21, 15 - MiG-23, 18 - Su-22, and 8 - Su-25 tank busters, but no Su-27s.
On the issue of Russian pilot, official AF media service reported that there are no military pilots in Angola of Russian origin.
However, Igor Valechenko (who was named to be the Su-27 pilot), could possibly be Ukranian citizen rather than Russian because for Africans all people from ex-Soviet Union are of Russian nationality.
As a reminder, UNITA announced hours earlier that one Su-27 was downed while approaching the runway for landing and about 0.5km from the airfield.