A single down-vote doesn't mean much. It doesn't say the question is that bad; differently it would have gotten more than 1 down-vote after being viewed 17 times.
As for the reason of the down-vote, I can only imagine what the reason could be.
I think it is because the question is about code you are looking for, but you don't show any attempt to write code yourself. This was probably enough for the user who down-voted to think that the question was a "write the code for me" question.
You show to understand enough the involved module to know what the module does, but it doesn't seem you have tried to write a custom module changing the code used by that module for its blocks. That leads to a question: Why didn't you try that step? If you didn't try that step, does that mean you don't know what function is responsible for showing a module's block?
You are on the right step, but trying to write a custom module with a changed version of that module's code would make the question even better. Showing code would help the users who answer to point you what you are doing wrong, and would incentive the other users to answer your question.
If you also show the original code (the relevant parts) and the code you tried, that would make a great question. As it is, the users should look at the code used by the mailchip module, see what it does, and try changing the code to see if it is possible to obtain a custom sign-in form. Users would still need to understand what you are trying to achieve, since you just said you want to get a custom sign-in form with more control over the look and feel, without exactly saying what that mean for you.
Then, there could be a simpler way to achieve what you are trying, and it could be that a user who knows well the mailchip module could point that out. Still, the question as I read it doesn't give any reason for the users to even try to find a solution.
Another reason to show the code is that the questions are thought to be helpful for future users, not just the one asking the question. With the code being shown, users would understand if the question is still relevant for them. It could be that future releases of the module use different code; if future readers see the module code changed, they will understand the question doesn't help anymore, and they could ask a new question for the new version of the module.