Does the toss in the T20 Blast give an advantage to the team who wins it? The % of games won by its winners over the years in the competition under its various names is shown in the chart below, together with the % of field first decisions.
In eight of the fifteen seasons its winners have won more games than they lost, in the other seven years the opposite. The numbers bounce around from one year to the next, in most years alternate above and below 50%, which they could be expected to do if winning the toss had essentially no effect on the outcomes of matches. Overall since 2003 the win % is 49.8; a pointer suggesting an answer to the question of quite possibly not.
As to the field first decisions made, the big shift to a preference for fielding first is striking; a previous bat/bowl ratio of around 70: 30 reversed over the present decade; conventional wisdoms about how to apply pressure on the opposition old and new, from which a reasonable inference is that captains, and the supporting cast of analysts as maybe, presumably do think there is, or at least might be, an advantage to exploit.
In general terms the overall numbers sit comfortably enough with the simple observation that winning the toss could be decisive in very tight matches, and with the view that a bit of luck should be a factor in sport but that the extent of any advantage be simply not large enough, often enough, to impact the win% figures. But the numbers do also prompt the question of why the current preference for fielding has gone as far as it has and also whether the decisions that are made are due for another shift?
The second chart shows the % of games won batting first and fielding first. In 2017 in rounded numbers the teams who won the toss chose to field 68% of the time, won 44% of the games, of which batting first they won 47%, fielding first 42%; which might not unreasonably prompt the thought that there were too many decisions to have a bowl first. Standing a year ago and looking back on the then previous 2016 season, the respective numbers are 69%, 57%, 54% and 58%, which might not unreasonably prompt the opposite thought.
In other words relying on the numbers for just one year could be rather misleading; in the early years of the competition, when the decisions made were towards batting first, there was for a time some supporting evidence of teams winning proportionately more often batting first. In the last few years it is hard, or at least harder, to say the same for the current preference for bowling first; not only do the win% tend to alternate from year to year, but also the breakdowns of wins when batting and bowling first as well.
It is, of course, possible that there could be a systematic advantage from the current bowl first decisions, even with the numbers above, but that the influence is conflated with other general and/or in-play influences. Some of which influences may also be measurable and possible to (statistically) model, but absent plausible evidence on this, the question is why is the toss anything more than a way of just starting matches?