So, you roll a surge, but it's not the kind of surge you need. Do they stay an unconverted surge die or do they convert to a blank?

It didn't really matter until Guardian came out....

While a friendly unit at range 1 is defending against a ranged attack, you may cancel up to X hit results. For each result canceled, roll one of your defense dice. Convert surges, then suffer 1 wound for each blank result.

c. Convert Defense Surges: The defender changes its defense surge (�) results to the result indicated on its unit card by turning the die. If no result is indicated, the defender changes the result to a blank.

Seems to indicate that surges are converted automatically, either to block OR blank as appropriate

3 minutes ago, Zrob314 said:

Do they stay an unconverted surge die or do they convert to a blank?

They become blank according to the Convert Surges steps of attacks (page 14).

Although the Surges and Guardian entries don't say so, the intent must be that you follow those steps as usual.