Should a capital letter be used after an ellipsis?

The first word after an ellipsis is capitalized if it begins a new grammatical sentence.¹

¹Chicago Manual of Style, 13.51.

Most writers would use a comma in OP's example, not an ellipsis. So the issue wouldn't arise there anyway. Probably the writer intended "Okay...blah blah" to make the reader "internally vocalise" it as "Okaaaaaay, blah blah".

In general, it really depends on whether you consider the ellipsis represents an "empty" pause at the end of a preceding sentence. If so, what follows is a new sentence, and it starts with a capital letter.

If you think the ellipsis represents a delay within an as-yet-incomplete sentence, but you've decided you don't want indicate that delay using some other punctuation (comma, semicolon, etc.), then just continue the sentence without a capital.

Effectively, it's partly the exact context, and partly stylistic preference. I'd say if in doubt, consider using a comma instead of an ellipsis. If that doesn't feel right, you should probably capitalise.