I have been thinking, and you have large dark eyes, when compared to other people.
I wasn't certain how to accurately translate 黒目がち, as there are several interpretations. However, most agree that 'large dark eyes' is accurate enough to be sufficient.
I've always thought your irises are larger compared to most people. They are, right?
I've always thought your pupils are bigger/larger than most people's. They are, aren't they?
For a long time I've been thinking your irises are larger than normal size for most people. Are they?
「黒目」は iris や pupil などと言えます。
「前からずっと思っていた」を、I've always thought .../ I've been thinking ...で表しています。
They are, right? や They are, aren't they? は「そうじゃない？」のニュアンスです。