Sivir doesn't have good synergy with Blitz, yes she has her ult which gives some synergy but other than that she's not that good with Blitz.
Her low attack range makes her susceptible to pressure from adc and enemy support, at the same time the only pressure Blitz can deal is by holding his hook and moving aggressivly. This means that you could easily start losing the lane just because your ADC is constantly being poked.
Then comes the second problem, her all-in power with Blitzcrank isn't insane. She doesn't have that bursty damage that other ADCs do (such as Lucian, Corki, Caitlyn etc) and her damage with only a Doran's isn't that good neither.
Playing against Sivir as Blitzcrank may sound like one extreme counter due to Sivir having a spellblock (E), but it isn't some really hard matchup but a skill matchup. Due to Sivir having a low autoattack range you can lock the wave slightly pushed onto your side.
This will cause Sivir to be forced to get closer to AA the caster creepline which is a perfect moment for your Mobi boots to come steaming out from the bush and flash E her. This forces her into either be quick and burn her spellshield and flash away OR she will eat your hook and ult.
Also I don't understand what Mordekaiser's doing at the synergy list with Blitz, Caitlyn/blitz have extreme synergy due to their combined pressure. You should swap Sivir with Janna instead.
The problem with comparing supports and adcs with each other is simply the combination of setups. For example, playing Blitz/Cait vs Morg/Sivir isn't very hard, but swapping Morgana for Thresh or Janna can become much harder.