This is actually the issue, you can't really do that. In order to smoothly blend, you need to know both speeds. You can't blend one side from 0 - 1 because that would go from a stopped animation to the full speed.
Ok i think i understand how you handle it.
In my solution it worked like this:
animation.play( normalized length of the clip from 0 - 1 )
And i knew the velocity of each motion and they were aligned so if one motion had the left-foot-hit-ground on 0.5 the other clips would also have left-foot-hit-ground on 0.5.
Now if i have a walk clip with 1m/s (if played from 0-1) and i needed 3m/s i would simply speed up the replay of this clip (*3), then if i say a run cycle fits better cause it has a original speed of 1.5m/s i would run this clip with *2 of the normal speed and crossblend the weights of both clips (the new clip with 0 at the beginning, _not_ the speed but the weight).
You are right thought that then your animations are not aligned any more. The best solution i saw for this problem was with Rune Johansens Locomotion solution for unity. Basically each leg is analyzed on it`s own for each walk-cycle. And the main states are stored (the positions in the cycle also normalized) for each leg, foot-hit, rest-position, foot-lift.. Then if your character has to walk 45° to the left a strafe-left and walk-forward motion are mixed and aligned based on the previous analyzed data.
You probably want to look into the source code and the thesis of this: