The rules provide no guidance, therefore it is your job as DM to make a ruling.
The obvious upper limit is that it has no effect and you can move at your full movement rate and the obvious lower limit is that it is totally debilitating and you can't move at all.
Did I say no guidance? Well, there is a little bit - crawling, swimming and climbing cost 2 for 1 as does difficult terrain and, of course, if these combine (e.g. crawling in difficult terrain) you are down to 4 for 1.
In making your ruling, ask yourself these questions:
- Is being blind more, less or the same disability as, say, crawling?
- Can a blinded creature take the Dash action?
- Does the creature need to make a check or saving throw to remain upright and unbludgeoned by tripping or walking into things. If so, what DC will you set (varying by the obstacles; perhaps DC10 for a flagged passage, DC25 for a natural cave)? Dexterity is the obvious stat; a Dexterity save or a Dexterity (Acrobatics) check? What are the consequences of failing; no progress, fall prone, damage?
In making your decision try to come to a consensus with your players about what seems reasonable but remember you make the final call. Whatever you decide, stick with it for consistency (unless it is obviously failing).
With respect to the suggestion spell itself:
The suggestion must be worded in such a manner
as to make the course of action sound reasonable.
Asking the creature to stab itself, throw itself onto
a spear, immolate itself, or do some other obviously
harmful act ends the spell.
Walking into the dark might be an "obviously