While I'm white, my natural hair colour is dark brown (almost black) and my beard started out blonde as well, so I know what you mean. It took about 1,5-2 years for it to get to that dark brown colour for me. However, I'm now 5,5 years on T and I still have a lot of light blonde hairs in my mustache and the hairs that connect to my goatee while the rest grows out solid dark brown.
My blonde streaks seems to stay like that. It almost looks like I'm greying very prematurely (I'm 28) cause of the strong contrast. But mostly it looks like my beard is much thinner than it actually is, cause the light blonde is very close to my skin colour. However I dye my beard and hair black to give it a more even tone and to look fuller, so I just notice it starts looking weird again when I've been slacking with the dyeing. It's actually quite common for cis guys to end up having multi-coloured beards, and is probably a reason why things like beard dyes exist.
All the men in my family/relatives are hairy and have thick facial hair. My father has always had a full beard since I was a kid, my uncle always has a strong 5 o'clock shadow, but I have no brothers. All my male relatives except from one kept their head hair too, so I really can't complain about my hair genes! So far I do have mostly quite thick facial hair as well, and it grows very evenly, however cheeks and front of my chin is still thin. My profile picture shows I have a low cheek-line. Most of that hair is very curly, thick and coarse but is finer in the more sparse areas. But I've also probably got a couple more years to give it, even though more hairs come in much slower now.
Most likely your facial hair will even out in colour over time, but some of the blonde could end up being persistent. However that happens to some cis guys too.
