Random walk terrain generator
Style

Variables and functions (except constructors) in JavaScript are normally written in camelCase with a leading lowercase letter. It's not mandatory, but you'll find it easier to work with others if you follow a language's conventions. There are many guides out there, but I mostly follow Google's JavaScript Style Guide.

Some of the variables do not change once set. These should use ALL_CAPS to denote that they are constants and probably assigned at the top.

Another point in the guide is to surround all operators and keywords with a space. Omitting the space between if, while, etc. and the opening parenthesis makes it harder to skim without syntax highlighting.

You can omit comments that merely restate the code in prose. At best they waste the readers time, and at worst they become misleading or incorrect when they aren't changed with the code. Good comments state programmer intent or clarify tricky algorithms. This comment adds no value.

// increment repeats
Repeats++;


Bug

As I noted in the comment, you have a typo bug:

if(BlockY &lt;= MinY || BlockX &gt;= MaxY) {
                     ^^^^^^


This is common when using similarly-named variables, but it's difficult to avoid doing so in rendering code.

Clarity

You can improve the performance of the random choice between -1, 0, and 1--and in my opinion make it more obvious--using a simpler equation without the array:

BlockX += floor(3 * random()) - 1;


You are using a while loop where a for loop would be more obvious. As written, you have to scan the whole body to see that Repeats only gets incremented once at the end. Use this instead:

for (var Repeats = 0; Repeats &lt; MaxRepeats; ++Repeats) { ... }