I have both as well and would agree with Ben’s assessment. They are very different whistles. And which suits best depends completely on your own preferences and purposes.
I first heard Cillian O’Briain play (the pipes) in 1980, his making and the vision it comes from is based in his expert musicianship, something he shares he shares with for example the Buckleys who make the Killarney, John Sindt or Colin Goldie. It may not matter to everybody, it matters to me and I feel the instruments coming from that place are player’s whistles. The O’Briain is my go to whistle for serious tunes around the house and I wouldn’t hesitate playing it in any context or setting at all.
The Blackbird I have was a pre-production prototype, I have since replaced the tube a few times (having now settled on a black Feadóg pro body that originally had a not very satisfying head attached), the filler in the head’s cavity developed a black mould so I replaced it and some of the plastic bits fell off in time and I re-glued them in a perhaps slightly different position (where I liked them best). The whistle is perhaps like Abraham Lincoln’s hammer, which, as the story goes, had it’s handle replaced three times and the head twice, in the hands of subsequent owners, but still goes as ‘Lincoln’s hammer’. I believe the whistle as it is now still functions within its originally envisaged parameters but with designs changing over time, I doubt you will get a whistle quite the same if you order one now. So there’s a major caveat.
That said, the Blackbird as I have it is light to the touch and nimble and requires you play it accordingly, if you do, it will reward you in kind. But the degree of breathcontrol it demands leaves it probably not the most suitable whistle for a beginner used to playing Susatos. It is voiced to operate on a narrow tonal band (I don’t know how to describe it otherwise) a fair amount of the original sound cut away, compared to untreated whistles, but not to such an extend it has lost all life but it is a ‘thinner’ sound. I have compared that type of sound as sounding like hearing a whistle from some distance or sounding like a whistle recorded with a degree of compression and equaliser applied. Originally it was sold as sounding ‘birdlike’ and that description gets repeated ad infinitum, I don’t know: last weekend a raven settled in a tree at the end of my driveway and got all excited, making a terrible racket. The whistle doesn’t sound like that at all, neither does it sound like the seabirds around when I walked the length of the Cliffs of Moher yesterday, or the tits, robins and finches that populate the apple tree in front of the house, where they visit the bird feeders in great numbers. Make of that what you will, when handled properly it plays like a whistle, handy enough, agile and lively (well, you’ll get out of it what you put in) . Mine in it’s current form is knocking about the house so I can pick it up when I get bored with the tellie or whatever I am doing or when I hear a tune what I want to pick up quickly. It gets played occasionally but never leaves the house where anyone can hear me play it.
That’s what I think anyway. I considered recording them both (I may yet) but am in doubt about the usefulness of it. I would suggest, again, you’re better off trying the whistles yourself and form your own opinion rather than processing impressions found on the internet and find yourself a whistle that suits your own tastes and abilities. Good luck.