That's a great tattoo! Your artist knew what they were doing.
A lot of artists won't do watercolor style tattoos because that style usually doesn't have a black outline or clients will insist that they don't want a black outline.
A black outline keeps the colors crisp and vibrant over time, rather than spreading into a blob of colors.
The part at the top, where there isn't a full outline, will probably fade nicely--kind of like how a real fire is less intense at the top of the flames and will probably give a cool effect to the piece.