Rather than the Bulgars that I could see pulling the same trick than Khazars (as in, converting superficially to Islam and more or less ignoring the over lordship of the Caliphate), the main issue is the Byzantine naval dominance.Why are the Arabs destined to take over the Balkans in Constantinople? Do we disregard the recent Bulgarians or are they converted already?
IOTL it's precisely why the 717-718 siege of Constantinople couldn't be won by Arabs, as the city was constantly supplied : forcing Maslamah to maintain a large army at an important cost for the Umayyad while still not being able to supply it.
Eventually that's the main issue on this region : Byzantine naval dominance is clear, as much as Arab reliance over Christian sailors on a less favorable position (both tactically and strategically).
Well, if Constantinople IS taken, it would mean a likely Islamization of Eastern Europe eventually : not only Russia but possibly on the Amber Road, Constantinople as an Islamic center being a certain focal point for much people around. It would certainly prevent Christian religious expansion not only in Balkans and Volga/Dniepr but possibly on Central Europe as well.2. Taking Constantinople opens opportunities of conversion in Eastern Europe but not Western Europe.