I can't comment on iambic meter because I struggled with it a lot while trying to write a sonnet....however thanks for the effort to translate the poem into English which is not easy especially by also maintaining a good rhythm...i especially liked the last stanza...
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I happen to have recently found at a used book store a collection called "Fifty Poems" of Pasternak translated by his sister, Lidia Pasternak Slater. Februrary is the first poem in the collection. She comments in the notes that before his numerous revisions of this poem, the "black spring" was referring to ."where showers are blacker still than ink and tears" and it referred to the slush "that giant black cauldrons in Moscow like those used for heating asphalt made the slush black when melted." Her final stanza reads

Beneath-the earth is black in puddles,
The wind with croaking screeches throbs,
And-the more randomly, the surer
Poems are forming out of sobs.

That can be compared to AZ Foreman's translation, the one in most used in anthologies:

A thaw patch blackens underfoot.
The wind is gutted with a scream.
True verses are the most haphazard,
Rhyming the heart out on a theme.

and yours:

There are black gaps in melted snow
and wind with croaking screeches throbs,
the more spontaneous the better
poems are composed.

Translations are the most difficult of tasks. I speak no Russian and sadly connot grasp the depth of the poem, it's inner music, rhyme scheme and all the rest. I like your translation, it is different in meter from the other two which have strictly counted syllable counts for each line, but all use a ABCB rhyming. Their ryhmes are very dirrect, yours are occassionally slant rhymes (speen/ sleet, throbs/composed) I have no way of knowing if this imitates the original. Your version seems a little more modern and free versed than the other 2 above. I don't think it's possible to translate a poem without putting yourself into it.