What is the past tense of bleak?

Bleak is not a verb, so it doesn't have a past tense.

This is the definition of bleak:

bleak 1

(blk)adj. bleak·er, bleak·est1.a. Gloomy and somber

b. Providing no encouragement; depressing:

2. Cold and cutting; raw

3. Exposed to the elements; unsheltered and barren